InputRange for data structure without order?

[Andy Valencia]

I have a Set data structure which has no concept of order; its 
members are stored and can be searched efficiently based on their 
hash.

I have a situation where chain()'ing them together would be 
convenient, but InputRange requires front() and popFront().  
There really _isn't_ a front, and creating a new Set with an 
element removed would be unpleasant.

What would be the best way to iterate across a list of such 
Set's?  For now I'm just going to write the explicit loop.

Thanks!
Andy