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diff --git a/docs/examples/pilib/rwlock.scala b/docs/examples/pilib/rwlock.scala
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+package examples.pilib;
+
+/**
+* From Pi to Scala: Semaphores, monitors, read/write locks.
+* Readers/writers locks.
+*/
+object rwlock {
+
+  import scala.concurrent.pilib._;
+
+  class Signal extends Chan[unit] {
+    def send = write(());
+    def receive = read;
+  }
+
+  class CountLock {
+    private val busy = new Signal;
+    def get = busy.send;
+    def release = busy.receive;
+    spawn < release >
+  }
+
+  /** A binary semaphore
+  */
+  class Lock {
+    private val busy = new Signal;
+    private val free = new Signal;
+    def get = busy.send;
+    def release = free.send;
+    spawn < (while (true) {
+      choice (
+        busy * (x => free.receive),
+	free * (x => ())
+      )
+    })
+    >
+  }
+
+  /** A monitor a la Java
+  */
+  class JavaMonitor {
+
+    private val lock = new Lock;
+
+    private var waiting: List[Signal] = Nil;
+
+    def Wait = {
+      val s = new Signal;
+      waiting = s :: waiting;
+      lock.release;
+      s.receive;
+      lock.get;
+    }
+
+    def Notify = {
+      if (!waiting.isEmpty) {
+	waiting.head.send;
+	waiting = waiting.tail;
+      }
+    }
+
+    def NotifyAll = {
+      while (!waiting.isEmpty) {
+	waiting.head.send;
+	waiting = waiting.tail;
+      }
+    }
+
+    def await(cond: => boolean): unit = while (false == cond) (Wait)
+  }
+
+  /*
+  class Buffer[a](size: Int) extends JavaMonitor with {
+    var in = 0, out = 0, n = 0;
+    val elems = new Array[a](size);
+    def put(x: a) = synchronized {
+      await(n < size);
+      elems(out) = x;
+      out = (out + 1) % size;
+    }
+    def get: a = synchronized {
+      await(n > 0);
+      val x = elems(in);
+      in = (in + 1) % size;
+      x
+    }
+  }
+  */
+
+  /** A readers/writers lock. */
+  trait ReadWriteLock {
+    def startRead: unit;
+    def startWrite: unit;
+    def endRead: unit;
+    def endWrite: unit;
+  }
+
+  /**
+  * A readers/writers lock, using monitor abstractions.
+  */
+  class ReadWriteLock1 extends JavaMonitor with ReadWriteLock {
+
+    private var nactive: int = 0;
+    private var nwriters: int = 0;
+
+    def status =
+      System.out.println(nactive + " active, " + nwriters + " writers");
+
+    def startRead = synchronized {
+      await(nwriters == 0);
+      nactive = nactive + 1;
+      status
+    }
+
+    def startWrite = synchronized {
+      nwriters = nwriters + 1;
+      await(nactive == 0);
+      nactive = 1;
+      status
+    }
+
+    def endRead = synchronized {
+      nactive = nactive - 1;
+      if (nactive == 0) NotifyAll;
+      status
+    }
+
+    def endWrite = synchronized {
+      nwriters = nwriters - 1;
+      nactive = 0;
+      NotifyAll;
+      status
+    }
+  }
+
+  /** A readers/writers lock, using semaphores
+  */
+  class ReadWriteLock2 extends ReadWriteLock {
+
+    private var rc: int = 0;  // reading readers
+    private var wc: int = 0;  // writing writers
+    private var rwc: int = 0; // waiting readers
+    private var wwc: int = 0; // waiting writers
+    private val mutex = new Lock;
+    private val rsem = new Lock;
+    private val wsem = new Lock;
+
+    def startRead = {
+      mutex.get;
+      if (wwc > 0 || wc > 0) {
+	rwc = rwc + 1;
+	mutex.release;
+	rsem.get;
+	rwc = rwc - 1;
+      }
+      rc = rc + 1;
+      if (rwc > 0) rsem.release;
+      mutex.release
+    }
+
+    def startWrite = {
+      mutex.get;
+      if (rc > 0 || wc > 0) {
+	wwc = wwc + 1;
+	mutex.release;
+	wsem.get;
+	wwc = wwc - 1;
+      }
+      wc = wc + 1;
+      mutex.release;
+    }
+
+    def endRead = {
+      mutex.get;
+      rc = rc - 1;
+      if (rc == 0 && wwc > 0) wsem.release;
+      mutex.release
+    }
+
+    def endWrite = {
+      mutex.get;
+      wc = wc - 1;
+      if (rwc > 0)
+	rsem.release
+      else if (wwc > 0) wsem.release;
+      mutex.release
+    }
+  }
+
+  /** A readers/writers lock, using channels, without priortities
+  */
+  class ReadWriteLock3 extends ReadWriteLock {
+
+    private val sr = new Signal;
+    private val er = new Signal;
+    private val sw = new Signal;
+    private val ew = new Signal;
+
+    def startRead = sr.send;
+    def startWrite = sw.send;
+    def endRead = er.send;
+    def endWrite = ew.send;
+
+    private def rwlock: unit = choice (
+      sr * (x => reading(1)),
+      sw * (x => { ew.receive; rwlock })
+    );
+
+    private def reading(n: int): unit = choice (
+      sr * (x => reading(n+1)),
+      er * (x => if (n == 1) rwlock else reading(n-1))
+    );
+
+    spawn < rwlock >;
+  }
+
+  /** Same, with sequencing
+  */
+  class ReadWriteLock4 extends ReadWriteLock {
+
+    private val rwlock = new ReadWriteLock3;
+
+    private val sr = new Signal;
+    private val ww = new Signal;
+    private val sw = new Signal;
+
+    def startRead = sr.send;
+    def startWrite = { ww.send; sw.send; }
+    def endRead = rwlock.endRead;
+    def endWrite = rwlock.endWrite;
+
+    private def queue: unit = choice (
+      sr * (x => { rwlock.startRead ; queue }),
+      ww * (x => { rwlock.startWrite; sw.receive; queue })
+    );
+
+    spawn < queue >;
+  }
+
+  /** Readwritelock where writers always have priority over readers
+  */
+  class ReadWriteLock5 extends ReadWriteLock {
+
+    private val sr = new Signal;
+    private val er = new Signal;
+    private val ww = new Signal;
+    private val sw = new Signal;
+    private val ew = new Signal;
+
+    def startRead = sr.send;
+    def startWrite = { ww.send; sw.send; }
+    def endRead = er.send;
+    def endWrite = ew.send;
+
+    private def Reading(nr: int, nw: int): unit =
+      if (nr == 0 && nw == 0)
+        choice (
+          sr * (x => Reading(1, 0)),
+          ww * (x => Reading(0, 1))
+        )
+      else if (nr == 0 && nw != 0) {
+        sw.receive;
+	Writing(nw);
+      }
+      else if (nr != 0 && nw == 0)
+        choice (
+          sr * (x => Reading(nr + 1, 0)),
+	  er * (x => Reading(nr - 1, 0)),
+          ww * (x => Reading(nr, 1))
+        )
+      else if (nr != 0 && nw != 0)
+        choice (
+          ww * (x => Reading(nr, nw + 1)),
+          er * (x => Reading(nr - 1, nw))
+        );
+
+    private def Writing(nw: int): unit  = choice (
+      ew * (x => Reading(0, nw - 1)),
+      ww * (x => Writing(nw + 1))
+    );
+
+    spawn < Reading(0, 0) >;
+
+  }
+
+  /**
+  * Main function.
+  */
+  def main(args: Array[String]): unit = {
+    val random = new java.util.Random();
+
+    def reader(i: int, rwlock: ReadWriteLock): unit = {
+      Thread.sleep(1 + random.nextInt(100));
+      System.err.println("Reader " + i + " wants to read.");
+      rwlock.startRead;
+      System.err.println("Reader " + i + " is reading.");
+      Thread.sleep(1 + random.nextInt(100));
+      rwlock.endRead;
+      System.err.println("Reader " + i + " has read.");
+      reader(i, rwlock)
+    }
+
+    def writer(i: int, rwlock: ReadWriteLock): unit = {
+      Thread.sleep(1 + random.nextInt(100));
+      System.err.println("Writer " + i + " wants to write.");
+      rwlock.startWrite;
+      System.err.println("Writer " + i + " is writing.");
+      Thread.sleep(1 + random.nextInt(100));
+      rwlock.endWrite;
+      System.err.println("Writer " + i + " has written.");
+      writer(i, rwlock)
+    }
+
+    val n = try { Integer.parseInt(args(0)) } catch { case _ => 0 }
+    if (n < 1 || 5 < n) {
+      Console.println("Usage: scala examples.pilib.rwlock <n> (n=1..5)");
+      exit
+    }
+    val rwlock = n match {
+      case 1 => new ReadWriteLock1
+      case 2 => new ReadWriteLock2
+      case 3 => new ReadWriteLock3
+      case 4 => new ReadWriteLock4
+      case 5 => new ReadWriteLock5
+    }
+    List.range(0, 5) foreach (i => spawn < reader(i, rwlock) >);
+    List.range(0, 5) foreach (i => spawn < writer(i, rwlock) >);
+  }
+
+}
+