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diff --git a/Chalice/tests/examples/CopyLessMessagePassing-with-ack.chalice b/Chalice/tests/examples/CopyLessMessagePassing-with-ack.chalice
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+// program inspired by "Proving Copyless Message Passing" (Villard, Lozes and Calcagno, APLAS 2009)
+
+// msg tag indicates what the type of the message
+// channel is freed by Getter when it completes
+// ack works, but an assume is needed and negative credits are sent over channels! 
+
+// Conjecture: it is ok to send debit for yourself over yourself.
+// Why: Suppose a channel that allows self-debt is involved in a deadlock. The either that channel is empty, which means there's no difference between the situation with or with self-debt. Or the channel is non-empty. This means that we can make progress by receiving the message stored in the channel! Does this make any sense?
+
+channel C(msg: int, n: Node) where 
+  (msg == 0 ==> credit(this, -1)) && // ack
+  (msg == 1 ==> n != null && acc(n.next) && acc(n.mu) && credit(this, -1)) && // cell
+  (msg == 2 ==> acc(this.mu, 50)); // done
+  
+
+class Node {
+  var next: Node;
+  
+  function length(): int
+    requires this.list;
+  {
+    unfolding this.list in 1 + (next == null ? 0 : next.length())
+  }
+  
+  predicate list {
+    acc(next) && acc(mu) && (next != null ==> next.list)
+  }
+}
+
+class Program {
+  method Putter(e: C, x0: Node) 
+    requires e!= null && acc(e.mu, 50) && e.mu == waitlevel && (x0 != null ==> x0.list) && (x0 != null ==> credit(e, - 1));
+  {
+    var x: Node := x0;
+    var t: Node;
+    
+    while(x != null) 
+      invariant (x != null ==> x.list) && acc(e.mu, 50) && credit(e, - 1);
+    {
+      unfold x.list;
+      t := x.next;
+      send e(1, x);
+      x := t;
+      var ack;
+      assume waitlevel << e.mu; // Chalice should be able to figure this out itself
+      receive ack, t := e;
+      if(ack != 2) { assume false; /* abort */ }
+    }
+    send e(2, null);
+  }  
+  
+  method Getter(f: C) 
+    requires f!= null && credit(f, 1) && acc(f.mu, 50) && waitlevel << f.mu;
+  {
+    var x: Node := null;
+    var msg := 1;
+    while(msg != 0)
+      invariant acc(f.mu, 50) && waitlevel << f.mu && (msg == 1 ==> credit(f, 1)) && (msg == 0 ==> acc(f.mu, 50));
+    {
+      receive msg, x := f;
+      if(msg == 0) {
+      }
+      if(msg == 1) {
+        free x;
+        send f(2, null);
+      }
+      if(msg != 0 || msg != 1) { assume false; /* abort */ }
+    }
+    free f; // close the channel
+  } 
+
+  method Main(x: Node)
+    requires x != null;
+    requires x.list;
+  {
+    var e := new C;
+    fork Putter(e, x);
+    fork Getter(e);
+  }  
+}
+
+
+