O(1) "popAny" for associative array?

[Andrew Klaassen via Digitalmars-d-learn]

In theory, it should be possible to do a "popFront" equivalent 
for a hash that has O(1) average complexity, so long as you don't 
care about order.  I.e., "give me any key from the hash, I don't 
care which one, and then delete it from the hash".  Is that 
correct?

If it is correct, is there any way to do it in D?
Do I assume correctly that "myarray.keys[0]" would not meet the 
O(1) requirement?

Thanks.

Andrew