exceptions

39 files changed, 174 insertions, 118 deletions

diff --git a/src/library/scala/Iterable.scala b/src/library/scala/Iterable.scala
index 1c1a3915de..2ab18f1f7b 100644
--- a/src/library/scala/Iterable.scala
+++ b/src/library/scala/Iterable.scala
@@ -39,7 +39,7 @@ object Iterable {
   /** The minimum element of a non-empty sequence of ordered elements */
   def min[A <% Ordered[A]](seq: Iterable[A]): A = {
     val xs = seq.elements
-    if (!xs.hasNext) Predef.error("min(<empty>)")
+    if (!xs.hasNext) throw new IllegalArgumentException("min(<empty>)")
     var min = xs.next
     while (xs.hasNext) {
       val x = xs.next
@@ -51,7 +51,7 @@ object Iterable {
   /** The maximum element of a non-empty sequence of ordered elements */
   def max[A <% Ordered[A]](seq: Iterable[A]): A = {
     val xs = seq.elements
-    if (!xs.hasNext) Predef.error("max(<empty>)")
+    if (!xs.hasNext) throw new IllegalArgumentException("max(<empty>)")
     var max = xs.next
     while (xs.hasNext) {
       val x = xs.next
diff --git a/src/library/scala/Iterator.scala b/src/library/scala/Iterator.scala
index 60774e9a05..54a3c92d62 100644
--- a/src/library/scala/Iterator.scala
+++ b/src/library/scala/Iterator.scala
@@ -25,7 +25,7 @@ object Iterator {
 
   def empty[a] = new Iterator[a] {
     def hasNext: Boolean = false
-    def next: a = Predef.error("next on empty iterator")
+    def next: a =  throw new java.util.NoSuchElementException("next on empty iterator")
   }
 
   def single[a](x: a) = new Iterator[a] {
@@ -33,7 +33,7 @@ object Iterator {
     def hasNext: Boolean = hasnext
     def next: a =
       if (hasnext) { hasnext = false; x }
-      else Predef.error("next on empty iterator")
+      else throw new java.util.NoSuchElementException("next on empty iterator")
   }
 
   def fromValues[a](xs: a*) = xs.elements
@@ -53,9 +53,9 @@ object Iterator {
       val end = if ((start + length) < xs.length) start else xs.length
       def hasNext: Boolean = i < end
       def next: a = if (hasNext) { val x = xs(i) ; i = i + 1 ; x }
-                    else Predef.error("next on empty iterator")
+                    else throw new java.util.NoSuchElementException("next on empty iterator")
       def head: a = if (hasNext) xs(i);
-                    else Predef.error("head on empty iterator")
+                    else throw new java.util.NoSuchElementException("head on empty iterator")
   }
 
   def fromString(str: String): Iterator[Char] =
@@ -103,10 +103,10 @@ object Iterator {
       def hasNext: Boolean = if (step > 0) i < end else i > end
       def next: Int =
         if (hasNext) { val j = i; i = i + step; j }
-        else Predef.error("next on empty iterator")
+        else throw new java.util.NoSuchElementException("next on empty iterator")
       def head: Int =
         if (hasNext) i
-        else Predef.error("head on empty iterator")
+        else throw new java.util.NoSuchElementException("head on empty iterator")
     }
   }
 
@@ -126,10 +126,10 @@ object Iterator {
       def hasNext: Boolean =  i < end
       def next: Int =
         if (i < end) { val j = i; i = step(i); j }
-        else Predef.error("next on empty iterator")
+        else throw new java.util.NoSuchElementException("next on empty iterator")
       def head: Int =
         if (i < end) i
-        else Predef.error("head on empty iterator")
+        else throw new java.util.NoSuchElementException("head on empty iterator")
     }
 
   /** Create an iterator with elements
@@ -203,7 +203,7 @@ trait Iterator[+A] {
     def hasNext = remaining > 0 && Iterator.this.hasNext
     def next: A =
       if (hasNext) { remaining = remaining - 1; Iterator.this.next }
-      else error("next on empty iterator")
+      else throw new java.util.NoSuchElementException("next on empty iterator")
   }
 
   /** Removes the first <code>n</code> elements from this iterator.
@@ -247,7 +247,7 @@ trait Iterator[+A] {
       else if (Iterator.this.hasNext) {
         cur = f(Iterator.this.next)
         next
-      } else Predef.error("next on empty iterator")
+      } else throw new java.util.NoSuchElementException("next on empty iterator")
   }
 
   /** Returns an iterator over all the elements of this iterator that
@@ -270,7 +270,7 @@ trait Iterator[+A] {
     def hasNext: Boolean = { skip; ahead || hasMore }
     def next: A =
       if (hasNext) { ahead = false; hd }
-      else Predef.error("next on empty iterator");
+      else throw new java.util.NoSuchElementException("next on empty iterator");
     def head: A = { skip; hd }
  }
 
diff --git a/src/library/scala/List.scala b/src/library/scala/List.scala
index 6ef56ce773..c0b5aedd88 100644
--- a/src/library/scala/List.scala
+++ b/src/library/scala/List.scala
@@ -472,7 +472,7 @@ sealed abstract class List[+a] extends Seq[a] with CaseClass {
     def hasNext: Boolean = !these.isEmpty
     def next: a =
       if (!hasNext)
-        error("next on empty Iterator")
+        throw new java.util.NoSuchElementException("next on empty Iterator")
       else {
         val result = these.head; these = these.tail; result
       }
@@ -491,7 +491,7 @@ sealed abstract class List[+a] extends Seq[a] with CaseClass {
    *  @throws <code>java.lang.RuntimeException</code> if the list is empty.
    */
   def init: List[a] =
-    if (isEmpty) error("Nil.init")
+    if (isEmpty) throw new UnsupportedOperationException("Nil.init")
     else {
       val b = new ListBuffer[a]
       var elem = head
@@ -510,7 +510,7 @@ sealed abstract class List[+a] extends Seq[a] with CaseClass {
    *  @throws <code>java.lang.RuntimeException</code> if the list is empty.
    */
   def last: a =
-    if (isEmpty) error("Nil.last")
+    if (isEmpty) throw new UnsupportedOperationException("Nil.last")
     else if (tail.isEmpty) head
     else tail.last
 
@@ -906,12 +906,12 @@ sealed abstract class List[+a] extends Seq[a] with CaseClass {
   }
 
   def reduceLeft[b >: a](f: (b, b) => b): b = this match {
-    case Nil => error("Nil.reduceLeft")
+    case Nil => throw new UnsupportedOperationException("Nil.reduceLeft")
     case x :: xs => ((xs: List[b]) foldLeft (x: b))(f)
   }
 
   def reduceRight[b >: a](f: (b, b) => b): b = this match {
-    case Nil => error("Nil.reduceRight")
+    case Nil => throw new UnsupportedOperationException("Nil.reduceRight")
     case x :: Nil => x: b
     case x :: xs => f(x, xs reduceRight f)
   }
@@ -1090,8 +1090,14 @@ sealed abstract class List[+a] extends Seq[a] with CaseClass {
 [SerialVersionUID(0 - 8256821097970055419L)]
 case object Nil extends List[Nothing] {
   override def isEmpty = true
-  def head: All = error("head of empty list")
-  def tail: List[Nothing] = error("tail of empty list")
+  /**
+   *  @throws java.util.NoSuchElementException
+   */
+  def head: All = throw new java.util.NoSuchElementException("head of empty list")
+  /**
+   *  @throws java.util.NoSuchElementException
+   */
+  def tail: List[Nothing] = throw new java.util.NoSuchElementException("tail of empty list")
 }
 
 /** A non empty list characterized by a head and a tail.
diff --git a/src/library/scala/Option.scala b/src/library/scala/Option.scala
index 1355b3ee24..636f32d59e 100644
--- a/src/library/scala/Option.scala
+++ b/src/library/scala/Option.scala
@@ -28,8 +28,11 @@ sealed abstract class Option[+A] extends Iterable[A] with CaseClass {
     case _ => false
   }
 
+  /**
+   *  @throws java.util.NoSuchElementException
+   */
   def get: A = this match {
-    case None => Predef.error("None.get")
+    case None => throw new java.util.NoSuchElementException("None.get")
     case Some(x) => x
   }
 
diff --git a/src/library/scala/Seq.scala b/src/library/scala/Seq.scala
index a52c54c020..8c9f896814 100644
--- a/src/library/scala/Seq.scala
+++ b/src/library/scala/Seq.scala
@@ -156,7 +156,7 @@ trait Seq[+A] extends AnyRef with PartialFunction[Int, A] with Iterable[A] {
         }
       }
     } else
-      Predef.error("cannot create subsequence");
+      throw new IllegalArgumentException("cannot create subsequence for "+from+","+len);
 
   /** Converts this sequence to a fresh Array */
   def toArray[B >: A]: Array[B] =
diff --git a/src/library/scala/Stream.scala b/src/library/scala/Stream.scala
index d5a8f3f2e5..4634bbdb19 100644
--- a/src/library/scala/Stream.scala
+++ b/src/library/scala/Stream.scala
@@ -24,8 +24,8 @@ object Stream {
 
   val empty: Stream[Nothing] = new Stream[Nothing] {
     override def isEmpty = true
-    def head: Nothing = error("head of empty stream")
-    def tail: Stream[Nothing] = error("tail of empty stream")
+    def head: Nothing = throw new java.util.NoSuchElementException("head of empty stream")
+    def tail: Stream[Nothing] = throw new java.util.NoSuchElementException("tail of empty stream")
     def printElems(buf: StringBuilder, prefix: String): StringBuilder = buf
   }
 
@@ -163,12 +163,12 @@ trait Stream[+a] extends Seq[a] {
   }
 
   def init: Stream[a] =
-    if (isEmpty) error("Stream.empty.init")
+    if (isEmpty) throw new java.util.NoSuchElementException("Stream.empty.init")
     else if (tail.isEmpty) Stream.empty
     else Stream.cons(head, tail.init)
 
   def last: a =
-    if (isEmpty) error("Stream.empty.last")
+    if (isEmpty) throw new java.util.NoSuchElementException("Stream.empty.last")
     else {
       def loop(s: Stream[a]): a = {
         if (s.tail.isEmpty) s.head
@@ -251,11 +251,11 @@ trait Stream[+a] extends Seq[a] {
     else f(head, tail.foldRight(z)(f))
 
   def reduceLeft[b >: a](f: (b, b) => b): b =
-    if (isEmpty) error("Stream.empty.reduceLeft")
+    if (isEmpty) throw new java.util.NoSuchElementException("Stream.empty.reduceLeft")
     else ((tail: Stream[b]) foldLeft (head: b))(f)
 
   def reduceRight[b >: a](f: (b, b) => b): b =
-    if (isEmpty) error("Stream.empty.reduceRight")
+    if (isEmpty) throw new java.util.NoSuchElementException("Stream.empty.reduceRight")
     else if (tail.isEmpty) head: b
     else f(head, tail.reduceRight(f))
 
diff --git a/src/library/scala/collection/Map.scala b/src/library/scala/collection/Map.scala
index bf51e021b9..8d5719f5c3 100644
--- a/src/library/scala/collection/Map.scala
+++ b/src/library/scala/collection/Map.scala
@@ -139,6 +139,6 @@ trait Map[A, +B] extends AnyRef
      *  @param key ...
      */
     def default(key: A): B =
-      error("key not found: " + key)
+      throw new java.util.NoSuchElementException("key not found: " + key)
 }
 
diff --git a/src/library/scala/collection/immutable/ListMap.scala b/src/library/scala/collection/immutable/ListMap.scala
index b4a6687cc9..6ead04fdfd 100644
--- a/src/library/scala/collection/immutable/ListMap.scala
+++ b/src/library/scala/collection/immutable/ListMap.scala
@@ -27,6 +27,8 @@ object ListMap {
 [serializable]
 class ListMap[A, B] extends AnyRef with Map[A, B] {
 
+  import java.util.NoSuchElementException
+
   /** This method returns a new ListMap instance mapping keys of the
    *  same type to values of type <code>C</code>.
    */
@@ -66,7 +68,7 @@ class ListMap[A, B] extends AnyRef with Map[A, B] {
     var that: ListMap[A,B] = ListMap.this;
     def hasNext = !that.isEmpty;
     def next: Pair[A,B] =
-      if (!hasNext) error("next on empty iterator")
+      if (!hasNext) throw new NoSuchElementException("next on empty iterator")
       else { val res = Pair(that.key, that.value); that = that.next; res }
   }
 
@@ -88,9 +90,9 @@ class ListMap[A, B] extends AnyRef with Map[A, B] {
 
   override def hashCode(): Int = 0
 
-  protected def key: A = error("empty map");
-  protected def value: B = error("empty map");
-  protected def next: ListMap[A, B] = error("empty map");
+  protected def key: A = throw new NoSuchElementException("empty map");
+  protected def value: B = throw new NoSuchElementException("empty map");
+  protected def next: ListMap[A, B] = throw new NoSuchElementException("empty map");
 
   [serializable]
   protected class Node(override protected val key: A, override protected val value: B)
diff --git a/src/library/scala/collection/immutable/ListSet.scala b/src/library/scala/collection/immutable/ListSet.scala
index 2924e4ef96..65d5c930ed 100644
--- a/src/library/scala/collection/immutable/ListSet.scala
+++ b/src/library/scala/collection/immutable/ListSet.scala
@@ -30,6 +30,8 @@ object ListSet {
 [serializable]
 class ListSet[A] extends AnyRef with Set[A] {
 
+  import java.util.NoSuchElementException
+
   /** Returns the number of elements in this set.
    *
    *  @return number of set elements.
@@ -58,13 +60,14 @@ class ListSet[A] extends AnyRef with Set[A] {
 
   /** Creates a new iterator over all elements contained in this set.
    *
+   *  @throws java.util.NoSuchElementException
    *  @return the new iterator
    */
   def elements: Iterator[A] = new Iterator[A] {
     var that: ListSet[A] = ListSet.this;
     def hasNext = !that.isEmpty;
     def next: A =
-      if (!hasNext) error("next on empty iterator")
+      if (!hasNext) throw new NoSuchElementException("next on empty iterator")
       else { val res = that.elem; that = that.next; res }
   }
 
@@ -78,8 +81,15 @@ class ListSet[A] extends AnyRef with Set[A] {
     } else
       false
 
-  protected def elem: A = error("Set has no elelemnts");
-  protected def next: ListSet[A] = error("Next of an empty set");
+  /**
+   *  @throws java.util.NoSuchElementException
+   */
+  protected def elem: A = throw new NoSuchElementException("Set has no elelemnts");
+
+  /**
+   *  @throws java.util.NoSuchElementException
+   */
+  protected def next: ListSet[A] = throw new NoSuchElementException("Next of an empty set");
 
   [serializable]
   protected class Node(override protected val elem: A) extends ListSet[A] {
diff --git a/src/library/scala/collection/immutable/Queue.scala b/src/library/scala/collection/immutable/Queue.scala
index 8e1730c74f..27bfdd4c32 100644
--- a/src/library/scala/collection/immutable/Queue.scala
+++ b/src/library/scala/collection/immutable/Queue.scala
@@ -24,6 +24,9 @@ object Queue {
  */
 [serializable]
 class Queue[+A](elem: A*) extends Seq[A] {
+
+  import java.util.NoSuchElementException
+
   protected val in: List[A] = Nil
   protected val out: List[A] = elem.elements.toList
 
@@ -38,7 +41,7 @@ class Queue[+A](elem: A*) extends Seq[A] {
    *
    *  @param  n index of the element to return
    *  @return   the element at position <code>n</code> in this queue.
-   *  @throws   <code>java.lang.Error</code> if the queue is too short.
+   *  @throws   <code>java.util.NoSuchElementException</code> if the queue is too short.
    */
   def apply(n: Int): A = {
     val len = out.length
@@ -46,7 +49,7 @@ class Queue[+A](elem: A*) extends Seq[A] {
     else {
       val m = n - len
       if (m < in.length) in.reverse.apply(m)
-      else error("index out of range")
+      else throw new NoSuchElementException("index out of range")
     }
   }
 
@@ -94,24 +97,26 @@ class Queue[+A](elem: A*) extends Seq[A] {
   /** Returns a tuple with the first element in the queue,
    *  and a new queue with this element removed.
    *
+   *  @throws NoSuchElementException
    *  @return the first element of the queue.
    */
   def dequeue: Pair[A, Queue[A]] = {
     val Pair(newOut, newIn) =
       if (out.isEmpty) Pair(in.reverse, Nil)
       else Pair(out, in);
-    if (newOut.isEmpty) error("queue empty")
+    if (newOut.isEmpty) throw new NoSuchElementException("queue empty")
     else Pair(newOut.head, mkQueue(newIn, newOut.tail))
   }
 
   /** Returns the first element in the queue, or throws an error if there
    *  is no element contained in the queue.
    *
+   *  @throws NoSuchElementException
    *  @return the first element.
    */
   def front: A =
     if (out.isEmpty) {
-      if (in.isEmpty) error("queue empty") else in.last
+      if (in.isEmpty) throw new NoSuchElementException("queue empty") else in.last
     } else
       out.head
 
diff --git a/src/library/scala/collection/immutable/Stack.scala b/src/library/scala/collection/immutable/Stack.scala
index b61c3ba16a..457efada8d 100644
--- a/src/library/scala/collection/immutable/Stack.scala
+++ b/src/library/scala/collection/immutable/Stack.scala
@@ -27,6 +27,8 @@ object Stack {
 [serializable]
 class Stack[+A] extends Seq[A] {
 
+  import java.util.NoSuchElementException
+
   /** Checks if this stack is empty.
    *
    *  @return true, iff there is no element on the stack.
@@ -69,13 +71,13 @@ class Stack[+A] extends Seq[A] {
    *
    *  @return the top element.
    */
-  def top: A = error("no element on stack")
+  def top: A = throw new NoSuchElementException("no element on stack")
 
   /** Removes the top element from the stack.
    *
    *  @return the new stack without the former top element.
    */
-  def pop: Stack[A] = error("no element on stack")
+  def pop: Stack[A] = throw new NoSuchElementException("no element on stack")
 
   /** Returns the n-th element of this stack. The top element has index
    *  0, elements below are indexed with increasing numbers.
@@ -83,7 +85,7 @@ class Stack[+A] extends Seq[A] {
    *  @param   n      the index number.
    *  @return the n-th element on the stack.
    */
-  def apply(n: Int): A = error("no element on stack")
+  def apply(n: Int): A = throw new NoSuchElementException("no element on stack")
 
   /** Returns an iterator over all elements on the stack. The iterator
    *  issues elements in the reversed order they were inserted into the
@@ -95,7 +97,7 @@ class Stack[+A] extends Seq[A] {
     var that: Stack[A] = Stack.this;
     def hasNext = !that.isEmpty;
     def next =
-      if (!hasNext) error("next on empty iterator")
+      if (!hasNext) throw new NoSuchElementException("next on empty iterator")
       else { val res = that.top; that = that.pop; res }
   }
 
diff --git a/src/library/scala/collection/immutable/Tree.scala b/src/library/scala/collection/immutable/Tree.scala
index 8e7a447df3..59284c3eb3 100644
--- a/src/library/scala/collection/immutable/Tree.scala
+++ b/src/library/scala/collection/immutable/Tree.scala
@@ -39,6 +39,8 @@
 
 package scala.collection.immutable
 
+import java.util.NoSuchElementException
+
 /** General Balanced Trees - highly efficient functional dictionaries.
  *
  *  <p>An efficient implementation of Prof. Arne Andersson's General
@@ -183,7 +185,7 @@ abstract class Tree[A <% Ordered[A], B]() extends AnyRef {
           iter = t.mk_iter(iter_tail)
           v
         case scala.Nil =>
-          error("next on empty iterator")
+          throw new NoSuchElementException("next on empty iterator")
       }
     }
 
@@ -269,8 +271,8 @@ private case class GBLeaf[A <% Ordered[A],B]() extends GBTree[A,B] {
   def count = Pair(1, 0)
   def isDefinedAt(key: A) = false
   def get(_key: A) = None
-  def apply(key: A) = error("key " + key + " not found")
-  def update(key: A, value: B) = error("key " + key + " not found")
+  def apply(key: A) = throw new NoSuchElementException("key " + key + " not found")
+  def update(key: A, value: B) = throw new NoSuchElementException("key " + key + " not found")
   def insert(key: A, value: B, s: Int): anInsertTree = {
     if (s == 0)
       INode(GBNode(key, value, this, this), 1, 1)
@@ -281,8 +283,8 @@ private case class GBLeaf[A <% Ordered[A],B]() extends GBTree[A,B] {
   def mk_iter(iter_tail: List[GBTree[A,B]]) = iter_tail
   def merge(larger: GBTree[A,B]) = larger
   def takeSmallest: Triple[A,B, GBTree[A,B]] =
-    error("Take Smallest on empty tree")
-  def delete(_key: A) = error("Delete on empty tree.")
+    throw new NoSuchElementException("takeSmallest on empty tree")
+  def delete(_key: A) = throw new NoSuchElementException("Delete on empty tree.")
   def balance(s: int) = this
   override def hashCode() = 0
 }
@@ -331,7 +333,7 @@ private case class GBNode[A <% Ordered[A],B](key: A,
     else if (newKey > key)
       bigger.insert(newKey, newValue, s / 2).insertRight(key, value, smaller)
     else
-      error("Key exists: " + newKey)
+      throw new NoSuchElementException("Key exists: " + newKey)
   }
 
   def toList(acc: List[Pair[A,B]]): List[Pair[A,B]] =
diff --git a/src/library/scala/collection/mutable/Buffer.scala b/src/library/scala/collection/mutable/Buffer.scala
index 31c7bd95c0..6e1d9e4475 100644
--- a/src/library/scala/collection/mutable/Buffer.scala
+++ b/src/library/scala/collection/mutable/Buffer.scala
@@ -199,23 +199,23 @@ trait Buffer[A] extends AnyRef
       case Start => prepend(elem)
       case End => append(elem)
       case Index(n) => insert(n, elem)
-      case _ => error("message " + cmd + " not understood")
+      case _ => throw new UnsupportedOperationException("message " + cmd + " not understood")
     }
     case Update(Pair(l, elem)) => l match {
       case Start => update(0, elem)
       case End => update(length - 1, elem)
       case Index(n) => update(n, elem)
-      case _ => error("message " + cmd + " not understood")
+      case _ => throw new UnsupportedOperationException("message " + cmd + " not understood")
     }
     case Remove(Pair(l, _)) => l match {
       case Start => remove(0)
       case End => remove(length - 1)
       case Index(n) => remove(n)
-      case _ => error("message " + cmd + " not understood")
+      case _ => throw new UnsupportedOperationException("message " + cmd + " not understood")
     }
     case Reset() => clear
     case s: Script[Pair[Location, A]] => s.elements foreach <<
-    case _ => error("message " + cmd + " not understood")
+    case _ => throw new UnsupportedOperationException("message " + cmd + " not understood")
   }
 
   /** Return a clone of this buffer.
@@ -229,7 +229,7 @@ trait Buffer[A] extends AnyRef
    *
    *  @return never.
    */
-  override def hashCode(): Int = error("unsuitable as hash key")
+  override def hashCode(): Int = throw new UnsupportedOperationException("unsuitable as hash key")
 
   /** Defines the prefix of the string representation.
    */
diff --git a/src/library/scala/collection/mutable/ListBuffer.scala b/src/library/scala/collection/mutable/ListBuffer.scala
index e1f1ff4d72..5df2be342a 100644
--- a/src/library/scala/collection/mutable/ListBuffer.scala
+++ b/src/library/scala/collection/mutable/ListBuffer.scala
@@ -119,7 +119,7 @@ final class ListBuffer[A] extends Buffer[A] {
   def apply(n: Int): A = try {
     start(n)
   } catch {
-    case ex: Error => throw new IndexOutOfBoundsException(n.toString())
+    case ex: Exception => throw new IndexOutOfBoundsException(n.toString())
   }
 
   /** Replace element at index <code>n</code> with the new element
@@ -146,7 +146,7 @@ final class ListBuffer[A] extends Buffer[A] {
       cursor.asInstanceOf[scala.::[A]].tl = newElem
     }
   } catch {
-    case ex: Error => throw new IndexOutOfBoundsException(n.toString())
+    case ex: Exception => throw new IndexOutOfBoundsException(n.toString())
   }
 
   /** Inserts new elements at the index <code>n</code>. Opposed to method
@@ -181,7 +181,7 @@ final class ListBuffer[A] extends Buffer[A] {
       }
     }
   } catch {
-    case ex: Error => throw new IndexOutOfBoundsException(n.toString())
+    case ex: Exception => throw new IndexOutOfBoundsException(n.toString())
   }
 
 
@@ -209,7 +209,7 @@ final class ListBuffer[A] extends Buffer[A] {
     }
     old
   } catch {
-    case ex: Error => throw new IndexOutOfBoundsException(n.toString())
+    case ex: Exception => throw new IndexOutOfBoundsException(n.toString())
   }
 
   /** Returns an iterator over all elements of this list.
@@ -222,7 +222,7 @@ final class ListBuffer[A] extends Buffer[A] {
     def hasNext: Boolean = !start.isEmpty && cursor != last
     def next: A =
       if (!hasNext) {
-        error("next on empty Iterator")
+        throw new java.util.NoSuchElementException("next on empty Iterator")
       } else {
         if (cursor == null) cursor = start else cursor = cursor.tail
         cursor.head
diff --git a/src/library/scala/collection/mutable/Map.scala b/src/library/scala/collection/mutable/Map.scala
index 78555a4e20..4b0c5e6b16 100644
--- a/src/library/scala/collection/mutable/Map.scala
+++ b/src/library/scala/collection/mutable/Map.scala
@@ -129,7 +129,7 @@ trait Map[A, B] extends AnyRef
     case Remove(Pair(k, _)) => this -= k
     case Reset() => clear
     case s: Script[Pair[A, B]] => s.elements foreach <<
-    case _ => error("message " + cmd + " not understood")
+    case _ => throw new UnsupportedOperationException("message " + cmd + " not understood")
   }
 
   /** Return a clone of this map.
@@ -143,7 +143,7 @@ trait Map[A, B] extends AnyRef
    *
    *  @return never.
    */
-  override def hashCode(): Int = error("unsuitable as hash key")
+  override def hashCode(): Int = throw new UnsupportedOperationException("unsuitable as hash key")
 
   /** Returns a string representation of this map which shows
    *  all the mappings.
diff --git a/src/library/scala/collection/mutable/Message.scala b/src/library/scala/collection/mutable/Message.scala
index 8c59ad2334..c5eae4eac7 100644
--- a/src/library/scala/collection/mutable/Message.scala
+++ b/src/library/scala/collection/mutable/Message.scala
@@ -76,5 +76,5 @@ class Script[A] extends ArrayBuffer[Message[A]] with Message[A] {
   }
 
   override def hashCode(): Int =
-    Predef.error("scripts are not suitable as hash keys")
+    throw new UnsupportedOperationException("scripts are not suitable as hash keys")
 }
diff --git a/src/library/scala/collection/mutable/MutableList.scala b/src/library/scala/collection/mutable/MutableList.scala
index 69e778ab66..6e5059d4a5 100644
--- a/src/library/scala/collection/mutable/MutableList.scala
+++ b/src/library/scala/collection/mutable/MutableList.scala
@@ -32,7 +32,7 @@ trait MutableList[A] extends Seq[A] with PartialFunction[Int, A] {
    *  yields an error if the element does not exist.
    */
   def apply(n: Int): A = get(n) match {
-    case None => error("element not found")
+    case None => throw new java.util.NoSuchElementException("element not found")
     case Some(value) => value
   }
 
diff --git a/src/library/scala/collection/mutable/ObservableBuffer.scala b/src/library/scala/collection/mutable/ObservableBuffer.scala
index 8112630da6..0ec53babd2 100644
--- a/src/library/scala/collection/mutable/ObservableBuffer.scala
+++ b/src/library/scala/collection/mutable/ObservableBuffer.scala
@@ -73,6 +73,8 @@ trait ObservableBuffer[A, This <: ObservableBuffer[A, This]] requires This
 
   abstract override def clear: Unit = {
     super.clear
-    publish(new Reset with Undoable { def undo: Unit = error("cannot undo") })
+    publish(new Reset with Undoable {
+      def undo: Unit = throw new UnsupportedOperationException("cannot undo")
+    })
   }
 }
diff --git a/src/library/scala/collection/mutable/ObservableMap.scala b/src/library/scala/collection/mutable/ObservableMap.scala
index da08fb95d7..f6713125d9 100644
--- a/src/library/scala/collection/mutable/ObservableMap.scala
+++ b/src/library/scala/collection/mutable/ObservableMap.scala
@@ -50,6 +50,8 @@ trait ObservableMap[A, B, This <: ObservableMap[A, B, This]] requires This
 
   abstract override def clear: Unit = {
     super.clear
-    publish(new Reset with Undoable { def undo: Unit = error("cannot undo") })
+    publish(new Reset with Undoable {
+      def undo: Unit = throw new UnsupportedOperationException("cannot undo")
+    })
   }
 }
diff --git a/src/library/scala/collection/mutable/ObservableSet.scala b/src/library/scala/collection/mutable/ObservableSet.scala
index 52b81b24d4..f437fa8cfa 100644
--- a/src/library/scala/collection/mutable/ObservableSet.scala
+++ b/src/library/scala/collection/mutable/ObservableSet.scala
@@ -38,6 +38,8 @@ trait ObservableSet[A, This <: ObservableSet[A, This]] requires This
 
   abstract override def clear: Unit = {
     super.clear
-    publish(new Reset with Undoable { def undo: Unit = error("cannot undo") })
+    publish(new Reset with Undoable {
+      def undo: Unit = throw new UnsupportedOperationException("cannot undo")
+    })
   }
 }
diff --git a/src/library/scala/collection/mutable/PriorityQueue.scala b/src/library/scala/collection/mutable/PriorityQueue.scala
index b6cb527d52..0c029dbe49 100644
--- a/src/library/scala/collection/mutable/PriorityQueue.scala
+++ b/src/library/scala/collection/mutable/PriorityQueue.scala
@@ -24,6 +24,8 @@ package scala.collection.mutable
 class PriorityQueue[A <% Ordered[A]] extends ResizableArray[A] {
   size = size + 1 // we do not use array(0)
 
+  import java.util.NoSuchElementException
+
   protected def fixUp(as: Array[A], m: Int): Unit = {
     var k: Int = m
     while ((k > 1) && (as(k / 2) < as(k))) {
@@ -89,6 +91,7 @@ class PriorityQueue[A <% Ordered[A]] extends ResizableArray[A] {
   /** Returns the element with the highest priority in the queue,
    *  and removes this element from the queue.
    *
+   *  @throws java.util.NoSuchElementException
    *  @return   the element with the highest priority.
    */
   def dequeue: A =
@@ -98,14 +101,14 @@ class PriorityQueue[A <% Ordered[A]] extends ResizableArray[A] {
       fixDown(array, 1, size - 1)
       array(size)
     } else
-      error("no element to remove from heap")
+      throw new NoSuchElementException("no element to remove from heap")
 
   /** Returns the element with the highest priority in the queue,
    *  or throws an error if there is no element contained in the queue.
    *
    *  @return   the element with the highest priority.
    */
-  def max: A = if (size > 1) array(1) else error("queue is empty")
+  def max: A = if (size > 1) array(1) else throw new NoSuchElementException("queue is empty")
 
   /** Removes all elements from the queue. After this operation is completed,
    *  the queue will be empty.
@@ -147,7 +150,7 @@ class PriorityQueue[A <% Ordered[A]] extends ResizableArray[A] {
    *
    *  @return never.
    */
-  override def hashCode(): Int = error("unsuitable as hash key")
+  override def hashCode(): Int = throw new UnsupportedOperationException("unsuitable as hash key")
 
   /** Returns a regular queue containing the same elements.
    */
diff --git a/src/library/scala/collection/mutable/Queue.scala b/src/library/scala/collection/mutable/Queue.scala
index d1a769b7eb..36fc82cf0d 100644
--- a/src/library/scala/collection/mutable/Queue.scala
+++ b/src/library/scala/collection/mutable/Queue.scala
@@ -58,11 +58,12 @@ class Queue[A] extends MutableList[A] {
   /** Returns the first element in the queue, and removes this element
    *  from the queue.
    *
+   *  @throws java.util.NoSuchElementException
    *  @return the first element of the queue.
    */
   def dequeue: A =
     if (first == null)
-      error("queue empty")
+      throw new java.util.NoSuchElementException("queue empty")
     else {
       val res = first.elem
       first = first.next
@@ -170,9 +171,10 @@ class Queue[A] extends MutableList[A] {
   /** The hashCode method always yields an error, since it is not
    *  safe to use mutable queues as keys in hash tables.
    *
+   *  @throws UnsupportedOperationException
    *  @return never.
    */
-  override def hashCode(): Int = error("unsuitable as hash key")
+  override def hashCode(): Int = throw new UnsupportedOperationException("unsuitable as hash key")
 
   /** Returns a textual representation of a queue as a string.
    *
diff --git a/src/library/scala/collection/mutable/Set.scala b/src/library/scala/collection/mutable/Set.scala
index 86b6268dcd..b64cd35db0 100644
--- a/src/library/scala/collection/mutable/Set.scala
+++ b/src/library/scala/collection/mutable/Set.scala
@@ -109,7 +109,7 @@ trait Set[A] extends AnyRef with collection.Set[A]
     case Remove(elem) => this -= elem
     case Reset() => clear
     case s: Script[A] => s.elements foreach <<
-    case _ => error("message " + cmd + " not understood")
+    case _ => throw new UnsupportedOperationException("message " + cmd + " not understood")
   }
 
   /** Return a clone of this set.
@@ -123,5 +123,5 @@ trait Set[A] extends AnyRef with collection.Set[A]
    *
    *  @return never.
    */
-  override def hashCode(): Int = error("unsuitable as hash key")
+  override def hashCode(): Int = throw new UnsupportedOperationException("unsuitable as hash key")
 }
diff --git a/src/library/scala/collection/mutable/SingleLinkedList.scala b/src/library/scala/collection/mutable/SingleLinkedList.scala
index d25de421ef..6a889fc49e 100644
--- a/src/library/scala/collection/mutable/SingleLinkedList.scala
+++ b/src/library/scala/collection/mutable/SingleLinkedList.scala
@@ -40,7 +40,7 @@ abstract class SingleLinkedList[A, This >: Null <: SingleLinkedList[A, This]]
 
   def apply(n: Int): A =
     if (n == 0) elem
-    else if (next == null) error("unknown element")
+    else if (next == null) throw new IndexOutOfBoundsException("unknown element")
     else next.apply(n - 1)
 
   def get(n: Int): Option[A] =
diff --git a/src/library/scala/collection/mutable/Stack.scala b/src/library/scala/collection/mutable/Stack.scala
index c33f85d447..cc9c262f00 100644
--- a/src/library/scala/collection/mutable/Stack.scala
+++ b/src/library/scala/collection/mutable/Stack.scala
@@ -20,6 +20,8 @@ package scala.collection.mutable
 [serializable, cloneable]
 class Stack[A] extends MutableList[A] {
 
+  import java.util.NoSuchElementException
+
   /** Checks if the stack is empty.
    *
    *  @return true, iff there is no element on the stack
@@ -59,11 +61,15 @@ class Stack[A] extends MutableList[A] {
    *  the element from the stack. An error is signaled if there is no
    *  element on the stack.
    *
+   *  @throws java.util.NoSuchElementException
    *  @return the top element
    */
-  def top: A = if (first == null) error("stack empty") else first.elem
+  def top: A = if (first == null) throw new NoSuchElementException("stack empty") else first.elem
 
   /** Removes the top element from the stack.
+   *
+   *  @throws java.util.NoSuchElementException
+   *  @return the top element
    */
   def pop: A =
     if (first != null) {
@@ -72,7 +78,7 @@ class Stack[A] extends MutableList[A] {
       len = len - 1;
       res
     } else
-      error("stack empty")
+      throw new NoSuchElementException("stack empty")
 
   /**
    * Removes all elements from the stack. After this operation completed,
@@ -112,7 +118,7 @@ class Stack[A] extends MutableList[A] {
    *
    *  @return never.
    */
-  override def hashCode(): Int = error("unsuitable as hash key")
+  override def hashCode(): Int = throw new UnsupportedOperationException("unsuitable as hash key")
 
   /** This method clones the stack.
    *
diff --git a/src/library/scala/concurrent/Actor.scala b/src/library/scala/concurrent/Actor.scala
index 22278df39e..793f2492ef 100644
--- a/src/library/scala/concurrent/Actor.scala
+++ b/src/library/scala/concurrent/Actor.scala
@@ -25,11 +25,11 @@ abstract class Actor extends Thread {
 
   def receive[a](f: PartialFunction[in.Message, a]): a =
     if (Thread.currentThread() == this) in.receive(f)
-    else error("receive called not on own process")
+    else throw new IllegalArgumentException("receive called not on own process")
 
   def receiveWithin[a](msec: long)(f: PartialFunction[in.Message, a]): a =
     if (Thread.currentThread() == this) in.receiveWithin(msec)(f)
-    else error("receiveWithin called not on own process")
+    else throw new IllegalArgumentException("receiveWithin called not on own process")
 
   private var pid: Pid = null
 
diff --git a/src/library/scala/concurrent/NameServer.scala b/src/library/scala/concurrent/NameServer.scala
index 6cdaca045c..2a08618508 100644
--- a/src/library/scala/concurrent/NameServer.scala
+++ b/src/library/scala/concurrent/NameServer.scala
@@ -24,13 +24,13 @@ object NameServer {
    *  @param proc ...
    */
   def register(name: Symbol, proc: Process) = {
-    if (names contains name) error("Name:" + name + " already registred")
+    if (names contains name) throw new IllegalArgumentException("Name:" + name + " already registred")
     names += name -> proc
   }
 
   def unregister(name: Symbol) =
     if (names contains name) names -= name
-    else error("Name:" + name + " not registred")
+    else throw new IllegalArgumentException("Name:" + name + " not registred")
 
   /**
    *  @param name ...
diff --git a/src/library/scala/concurrent/Process.scala b/src/library/scala/concurrent/Process.scala
index fce91b50b8..565a2e28d7 100644
--- a/src/library/scala/concurrent/Process.scala
+++ b/src/library/scala/concurrent/Process.scala
@@ -36,11 +36,14 @@ object Process {
   def receiveWithin[a](msec: long)(f: PartialFunction[MailBox#Message, a]): a =
     self.receiveWithin(msec)(f)
 
+  /**
+   *  @throws UnsupportedOperationException
+   */
   def self: Process =
     if (Thread.currentThread().isInstanceOf[Process])
       Thread.currentThread().asInstanceOf[Process]
     else
-      error("Self called outside a process")
+      throw new UnsupportedOperationException("Self called outside a process")
 
   def exit(p: Process, reason: AnyRef) =
     p.exit(reason)
diff --git a/src/library/scala/io/BytePickle.scala b/src/library/scala/io/BytePickle.scala
index ded1bbafcc..19d7b99bd5 100644
--- a/src/library/scala/io/BytePickle.scala
+++ b/src/library/scala/io/BytePickle.scala
@@ -151,7 +151,7 @@ object BytePickle {
         case Ref() =>
           val res2 = unat.appU(res._2)  // read location
           upe.get(res2._1) match {     // lookup value in unpickler env
-            case None => error("invalid unpickler environment"); return null
+            case None => throw new IllegalArgumentException("invalid unpickler environment"); return null
             case Some(v) => return Pair(v.asInstanceOf[a], new UnPicklerState(res2._2, upe))
           }
       }
@@ -160,14 +160,14 @@ object BytePickle {
 
   def ulift[t](x: t): PU[t] = new PU[t] {
     def appP(a: t, state: Array[byte]): Array[byte] =
-      if (x != a) { error("value to be pickled (" + a + ") != " + x); state }
+      if (x != a) { throw new IllegalArgumentException("value to be pickled (" + a + ") != " + x); state }
       else state;
     def appU(state: Array[byte]) = Pair(x, state);
   }
 
   def lift[t](x: t): SPU[t] = new SPU[t] {
     def appP(a: t, state: PicklerState): PicklerState =
-      if (x != a) { /*error("value to be pickled (" + a + ") != " + x);*/ state }
+      if (x != a) { /*throw new IllegalArgumentException("value to be pickled (" + a + ") != " + x);*/ state }
       else state;
     def appU(state: UnPicklerState) = Pair(x, state);
   }
diff --git a/src/library/scala/io/Position.scala b/src/library/scala/io/Position.scala
index 6537e54b7b..8993e4a812 100644
--- a/src/library/scala/io/Position.scala
+++ b/src/library/scala/io/Position.scala
@@ -69,11 +69,11 @@ object Position {
   final def encode(line: Int, column: Int): Int = {
     var line1, column1 = 0
     if (line < 0)
-      error(line + " < 0")
+      throw new IllegalArgumentException(line + " < 0")
     if ((line == 0) && (column != 0))
-      error(line + "," + column + " not allowed")
+      throw new IllegalArgumentException(line + "," + column + " not allowed")
     if (column < 0)
-      error(line + "," + column + " not allowed")
+      throw new IllegalArgumentException(line + "," + column + " not allowed")
 
     {if (line >= LINE_MASK) {
       line1 = LINE_MASK
diff --git a/src/library/scala/runtime/RichString.scala b/src/library/scala/runtime/RichString.scala
index 6744d302a3..d6195e034b 100755
--- a/src/library/scala/runtime/RichString.scala
+++ b/src/library/scala/runtime/RichString.scala
@@ -56,7 +56,7 @@ final class RichString(s: String) {
     var index = 0
     def hasNext: Boolean = index <= len
     def next: String = {
-      if (index >= len) Predef.error("next on empty iterator")
+      if (index >= len) throw new java.util.NoSuchElementException("next on empty iterator")
       val start = index
       while (index < len && !isLineBreak(apply(index))) index = index + 1
       index = index + 1
diff --git a/src/library/scala/util/automata/BaseBerrySethi.scala b/src/library/scala/util/automata/BaseBerrySethi.scala
index 522e3dc903..7a7a6c642f 100644
--- a/src/library/scala/util/automata/BaseBerrySethi.scala
+++ b/src/library/scala/util/automata/BaseBerrySethi.scala
@@ -64,7 +64,7 @@ abstract class BaseBerrySethi {
       tmp
     case Star(t) => compFirst(t)
     case _ =>
-      error("unexpected pattern " + Platform.getClass(r))
+      throw new IllegalArgumentException("unexpected pattern " + Platform.getClass(r))
   }
 
   /** computes last( r ) for the regexp r */
@@ -89,7 +89,7 @@ abstract class BaseBerrySethi {
       tmp
     case Star(t)  => compLast(t)
     case _        =>
-      error("unexpected pattern " + Platform.getClass(r))
+      throw new IllegalArgumentException("unexpected pattern " + Platform.getClass(r))
   }
 
   /** Starts from the right-to-left
@@ -170,7 +170,7 @@ abstract class BaseBerrySethi {
         first
 
       case _ =>
-        error("unexpected pattern: " + Platform.getClass(r))
+        throw new IllegalArgumentException("unexpected pattern: " + Platform.getClass(r))
     }
   }
 
@@ -193,7 +193,7 @@ abstract class BaseBerrySethi {
     case Star(t) =>
       traverse(t)
     case _ =>
-      error("unexp pattern " + Platform.getClass(r))
+      throw new IllegalArgumentException("unexp pattern " + Platform.getClass(r))
   }
 
 }
diff --git a/src/library/scala/xml/Elem.scala b/src/library/scala/xml/Elem.scala
index 1cf463051e..96b571c505 100644
--- a/src/library/scala/xml/Elem.scala
+++ b/src/library/scala/xml/Elem.scala
@@ -29,21 +29,15 @@ case class Elem(override val prefix: String,
                 val child: Node*) extends Node {
 
   if (prefix != null && 0 == prefix.length())
-    scala.Predef.error("prefix of zero length, use null instead");
+    throw new IllegalArgumentException("prefix of zero length, use null instead");
 
   if (null == scope)
-    scala.Predef.error("scope is null");
+    throw new IllegalArgumentException("scope is null, try xml.TopScope for empty scope");
 
   //@todo: copy the children,
   //  setting namespace scope if necessary
   //  cleaning adjacent text nodes if necessary
 
-  //final val namespaceIntern     = namespace$$.intern();
-  //final def namespace  = namespaceIntern;
-
-  //final val labelIntern = label$$.intern();
-  //final def label       = labelIntern;
-
   final override def typeTag$: Int = 0;
 
   override def hashCode(): Int = {
diff --git a/src/library/scala/xml/Group.scala b/src/library/scala/xml/Group.scala
index 8a7cc03da2..c5f7911138 100644
--- a/src/library/scala/xml/Group.scala
+++ b/src/library/scala/xml/Group.scala
@@ -31,25 +31,35 @@ case class Group(val nodes: Seq[Node]) extends Node {
     case _           => false;
   }
 
-  /** always null */
+  /**
+   * @throws UnsupportedOperationException (always)
+   */
   final def label =
-    error("class Group does not support method 'label'")
+    throw new UnsupportedOperationException("class Group does not support method 'label'")
 
-  /** always empty */
+  /**
+   * @throws UnsupportedOperationException (always)
+   */
   final override def attributes =
-    error("class Group does not support method 'attributes'")
+    throw new UnsupportedOperationException("class Group does not support method 'attributes'")
 
-  /** always null */
+  /**
+   * @throws UnsupportedOperationException (always)
+   */
   final override def namespace =
-    error("class Group does not support method 'namespace'")
+    throw new UnsupportedOperationException("class Group does not support method 'namespace'")
 
-  /** always empty */
+  /**
+   * @throws UnsupportedOperationException (always)
+   */
   final override def child =
-    error("class Group does not support method 'child'")
+    throw new UnsupportedOperationException("class Group does not support method 'child'")
 
-  /** returns text, with some characters escaped according to XML spec */
+  /**
+   * @throws UnsupportedOperationException (always)
+   */
   def toString(sb: StringBuilder) =
-    error("class Group does not support method toString(StringBuilder)")
+    throw new UnsupportedOperationException("class Group does not support method toString(StringBuilder)")
 
   override def text = { // same impl as NodeSeq
     val sb = new StringBuilder()
diff --git a/src/library/scala/xml/NamespaceBinding.scala b/src/library/scala/xml/NamespaceBinding.scala
index 7ad05dfcc5..cccfa893c1 100644
--- a/src/library/scala/xml/NamespaceBinding.scala
+++ b/src/library/scala/xml/NamespaceBinding.scala
@@ -29,7 +29,7 @@ class NamespaceBinding(val prefix: String,
   private val serialVersionUID = 0 - 2518644165573446725L
 
   if (null != prefix && 0 == prefix.length())
-    Predef.error("zero length prefix not allowed")
+    throw new IllegalArgumentException("zero length prefix not allowed")
 
   def getURI(_prefix: String): String =
     if (prefix == _prefix) uri else parent.getURI(_prefix)
diff --git a/src/library/scala/xml/dtd/Decl.scala b/src/library/scala/xml/dtd/Decl.scala
index eaa545b98b..862c48fa90 100644
--- a/src/library/scala/xml/dtd/Decl.scala
+++ b/src/library/scala/xml/dtd/Decl.scala
@@ -120,7 +120,7 @@ case class IntDef(value:String) extends EntityDef {
     while( ix != -1) {
       val iz = tmp.indexOf(';', ix);
       if(iz == -1 && iz == ix + 1)
-        error("no % allowed in entity value, except for parameter-entity-references");
+        throw new IllegalArgumentException("no % allowed in entity value, except for parameter-entity-references");
       else {
         val n = tmp.substring(ix, iz);
 
diff --git a/src/library/scala/xml/parsing/MarkupParser.scala b/src/library/scala/xml/parsing/MarkupParser.scala
index b78a3d0969..c3a69b555a 100644
--- a/src/library/scala/xml/parsing/MarkupParser.scala
+++ b/src/library/scala/xml/parsing/MarkupParser.scala
@@ -1098,8 +1098,10 @@ trait MarkupParser requires (MarkupParser with MarkupHandler) extends AnyRef wit
       else
         null;
       new PublicID(pubID, sysID);
-    } else
-      error("PUBLIC or SYSTEM expected");
+    } else {
+      reportSyntaxError("PUBLIC or SYSTEM expected");
+      error("died parsing notationdecl")
+    }
     xSpaceOpt
     xToken('>')
     handle.notationDecl(notat, extID)
diff --git a/src/library/scala/xml/parsing/ValidatingMarkupHandler.scala b/src/library/scala/xml/parsing/ValidatingMarkupHandler.scala
index 8c1d0a5767..a233cc527c 100644
--- a/src/library/scala/xml/parsing/ValidatingMarkupHandler.scala
+++ b/src/library/scala/xml/parsing/ValidatingMarkupHandler.scala
@@ -36,7 +36,7 @@ abstract class ValidatingMarkupHandler extends MarkupHandler with Logged {
     val res = decl.contentModel.validate(ns);
     Console.println("res = "+res);
     if(!res)
-      error("invalid!");
+      //error("invalid!");
   }
   */
 
diff --git a/src/library/scala/xml/transform/BasicTransformer.scala b/src/library/scala/xml/transform/BasicTransformer.scala
index ca9cd70a9d..4d3bfbe252 100644
--- a/src/library/scala/xml/transform/BasicTransformer.scala
+++ b/src/library/scala/xml/transform/BasicTransformer.scala
@@ -104,7 +104,7 @@ abstract class BasicTransformer extends Function1[Node,Node] {
   def apply(n: Node): Node = {
     val seq = transform(n)
     if (!single(seq))
-      error("transform must return single node for root");
+      throw new UnsupportedOperationException("transform must return single node for root");
     else seq.elements.next
   }
 }