--- a/Behavior.st	Thu Jun 07 20:29:37 2018 +0200
+++ b/Behavior.st	Thu Jun 07 20:29:50 2018 +0200
@@ -4005,6 +4005,16 @@
     "
 !
 
+mutableClass
+    "Return a version of me with mutable instances.
+     Only redefined in the immutable collections 
+     (of which instances are created by the compiler)"
+
+    ^ self
+
+    "Created: / 07-06-2018 / 20:26:46 / Claus Gittinger"
+!
+
 subclasses
     "return a collection of the direct subclasses of the receiver"