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sets and bags

The dylan book states:
'all collections in Dylan are "keyed" collections...this choice precludes
pure sets...although it is straightforward to simply ignore the keys for a
Why was this particular choice made?  There does not appear to be anything
in the interface for <collection> which would preclude pure Sets, or Bags
for that matter.
Smalltalk provides Sets and they are very useful (Bags less so in my
opinion).  Set semantics are often what is needed and using a keyed
collection seems wasteful of space.  It also feels wrong in the sense that
a keyed collection is not the abstraction desired.