collection-fold-right for unordered collections

[Jim White <jim@xxxxxxxxxxxxxx>]

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 
multiple times.

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.

Jim
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