--- a/Collection.st	Tue Mar 07 21:01:42 2017 +0100
+++ b/Collection.st	Thu Mar 09 10:25:19 2017 +0100
@@ -753,6 +753,14 @@
     self at:aKey put:(self at:aKey ifAbsent:[0])+count.
 !
 
+keys
+    "return the keys of the collection."
+
+    ^ self subclassResponsibility
+
+    "Created: / 09-03-2017 / 10:24:18 / cg"
+!
+
 keysSorted
     "return the keys as a sorted sequenceable collection.
      Some collections (which keep their keys already sorted) may