[Pharo-project] Permutations and Combinations in SequenceableCollection

Hi, I'm trying to generate the permutations of a sequence of numbers. I've found # permutationsDo: aBlock, but this generated the permutation of the entire sequence. I want to do it in groups of 2. The #combinations: anInteger atATimeDo: aBlock method does what I need for combinations, but I cannot find the equivalent for permutations. Does something like this exist in the standard library, or in an external package. -- -JT