--- a/Collection.st	Wed Feb 15 22:29:42 2017 +0100
+++ b/Collection.st	Thu Feb 16 13:45:01 2017 +0100
@@ -1,3 +1,5 @@
+"{ Encoding: utf8 }"
+
 "
  COPYRIGHT (c) 1989 by Claus Gittinger
               All Rights Reserved
@@ -343,6 +345,7 @@
     ^ self
 ! !
 
+
 !Collection methodsFor:'Compatibility-ANSI'!
 
 identityIncludes:anObject
@@ -1426,6 +1429,23 @@
         ret at:i put:(self removeLast).
     ].
     ^ ret
+!
+
+testAndAdd:anElement
+    "add the argument, anObject to the receiver.
+     Answer true, if the element did already exist in the collection,
+     false otherwise.
+
+     WARNING: do not add elements while iterating over the receiver.
+              Iterate over a copy to do this."
+
+    (self includes:anElement) ifTrue:[
+        ^ true.
+    ].
+    self add:anElement.
+    ^ false.
+
+    "Created: / 16-02-2017 / 13:41:58 / stefan"
 ! !
 
 !Collection methodsFor:'bulk operations'!
@@ -4351,7 +4371,7 @@
     aStream nextPut:$)
 
     "
-     #(1 2 3 'hello' $a $ü) printOn:Transcript
+     #(1 2 3 'hello' $a $ü) printOn:Transcript
      (Array new:100000) printOn:Transcript
      (Array new:100000) printOn:Stdout
      (Array new:100000) printString size
@@ -5708,7 +5728,7 @@
 includesAll:aCollection
     "return true if the receiver includes all elements of
      the argument, aCollection; false if any is missing.
-     Notice: this method has O² runtime behavior and may be
+     Notice: this method has O² runtime behavior and may be
              slow for big receivers/args.
              Think about using a Set, or Dictionary."
 
@@ -5728,7 +5748,7 @@
      Return false if it includes none.
      Uses #= (value compare)
      Notice:
-        this method has O² runtime behavior for some subclasses and may be slow for big receivers/args.
+        this method has O² runtime behavior for some subclasses and may be slow for big receivers/args.
         Think about using a Set or Dictionary.
         Some speedup is also possible, by arranging highly
         probable elements towards the beginning of aCollection, to avoid useless searches.
@@ -5805,7 +5825,7 @@
      Return false if it includes none.
      Use identity compare for comparing.
      Notice:
-        this method has O² runtime behavior for some subclasses and may be slow for big receivers/args.
+        this method has O² runtime behavior for some subclasses and may be slow for big receivers/args.
         Think about using a Set or Dictionary.
         Some speedup is also possible, by arranging highly
         probable elements towards the beginning of aCollection, to avoid useless searches."