Comment #1 on issue 3533 by stephane.ducasse: add Dictionary>>at:ifAbsentPutValue:
http://code.google.com/p/pharo/issues/detail?id=3533
at: key ifAbsentPutValue: anObject
"Return the value at the given key.
If key is not included in the receiver store the new value."
^ self at: key ifAbsent: [self at: key put: anObject ]
