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diff --git a/src/library/scala/collection/immutable/ListSet.scala b/src/library/scala/collection/immutable/ListSet.scala
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+++ b/src/library/scala/collection/immutable/ListSet.scala
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+/*                     __                                               *\
+**     ________ ___   / /  ___     Scala API                            **
+**    / __/ __// _ | / /  / _ |    (c) 2003-2005, LAMP/EPFL             **
+**  __\ \/ /__/ __ |/ /__/ __ |                                         **
+** /____/\___/_/ |_/____/_/ | |                                         **
+**                          |/                                          **
+\*                                                                      */
+
+// $Id$
+
+package scala.collection.immutable;
+
+
+object ListSet {
+    /** constructs an empty ListSet
+     */
+    def Empty[A] = new ListSet[A];
+}
+
+
+/** This class implements immutable sets using a list-based data
+ *  structure. Instances of <code>ListSet</code> represent
+ *  empty sets; they can be either created by calling the constructor
+ *  directly, or by applying the function <code>ListSet.Empty</code>.
+ *
+ *  @author  Matthias Zenger
+ *  @version 1.0, 09/07/2003
+ */
+[serializable]
+class ListSet[A] extends AnyRef with Set[A] {
+
+    /** Returns the number of elements in this set.
+     *
+     *  @return number of set elements.
+     */
+    def size: Int = 0;
+
+    /** Checks if this set contains element <code>elem</code>.
+     *
+     *  @param  elem    the element to check for membership.
+     *  @return true, iff <code>elem</code> is contained in this set.
+     */
+    def contains(elem: A): Boolean = false;
+
+    /** This method creates a new set with an additional element.
+     */
+    def +(elem: A): ListSet[A] = new Node(elem);
+
+    /** <code>-</code> can be used to remove a single element from
+     *  a set.
+     */
+    def -(elem: A): ListSet[A] = this;
+
+    /** Creates a new iterator over all elements contained in this
+     *  object.
+     *
+     *  @return the new iterator
+     */
+    def elements: Iterator[A] = toList.elements;
+
+    /** Transform this set into a list of all elements.
+     *
+     *  @return  a list which enumerates all elements of this set.
+     */
+    override def toList: List[A] = Nil;
+
+    /** Compares two sets for equality.
+     *   Two set are equal iff they contain the same elements.
+     */
+    override def equals(obj: Any): Boolean =
+        if (obj.isInstanceOf[scala.collection.Set[A]]) {
+            val that = obj.asInstanceOf[scala.collection.Set[A]];
+            if (size != that.size) false else toList.forall(that.contains);
+        } else
+            false;
+
+    override def hashCode(): Int = 0;
+
+    protected class Node(elem: A) extends ListSet[A] {
+        /** Returns the number of elements in this set.
+         *
+         *  @return number of set elements.
+         */
+        override def size = ListSet.this.size + 1;
+
+        /** Checks if this set is empty.
+         *
+         *  @return true, iff there is no element in the set.
+         */
+        override def isEmpty: Boolean = false;
+
+        /** Checks if this set contains element <code>elem</code>.
+         *
+         *  @param  elem    the element to check for membership.
+         *  @return true, iff <code>elem</code> is contained in this set.
+         */
+        override def contains(e: A) = (e == elem) || ListSet.this.contains(e);
+
+        /** This method creates a new set with an additional element.
+         */
+        override def +(e: A): ListSet[A] = if (contains(e)) this else new Node(e);
+
+        /** <code>-</code> can be used to remove a single element from
+         *  a set.
+         */
+        override def -(e: A): ListSet[A] = if (e == elem) ListSet.this else {
+          val tail = ListSet.this - e; new tail.Node(elem)
+        }
+
+        /** Transform this set into a list of all elements.
+         *
+         *  @return  a list which enumerates all elements of this set.
+         */
+        override def toList: List[A] = elem :: ListSet.this.toList;
+
+        override def hashCode(): Int = elem.hashCode() + ListSet.this.hashCode();
+    }
+}