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collection-fold-right for unordered collections
I'm thinking that unordered collections are not quite fully specified.
A key question is whether collection-fold-left will return the elements
of the collection in the same (unspecified) order or not when used
It seems to me that either unordered collections should either always
enumerate in the same order as long as they are not modified, or there
needs to be some way to distinguish unordered collections that make no
representation about repeating the enumeration order.
And if an unordered collection *will* preserve the enumeration order
(which would be typical for many straightforward implementations) then
collection-fold-right should be defined to enumerate in the reverse of
that same unspecified (but invariant-while-not-modified) order.
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