Since multiple people seem don't grok my example, let me simplify it a bit:

Suppose you have a special collection of objects over which you can iterate.

The items in that collection could be anything - you don't really care
on almost any of them, except those ones which is also a special
collections of same kind as a container.

so, then you could have a method, which by iterating through your
special collection, ignores all objects except those who
also can be iterated in same manner:

MySpecialCollection>>mySpecialCollect: aBlock
     ^ self collect: [:each |
          each isMySpecialCollection ifTrue: [ each mySpecialCollect:
aBlock] ifFalse: [each]
      ]

-- 
Best regards,
Igor Stasenko AKA sig.