I would suggest that educated_foo's (++) is the most portable,readable one with no dependencies.

That isn't the OP's criteria ("I am trying to find the fastest (not necessarily most elegant) way"). I think both his and mine are similarly fast, however.

I specifically avoided undef @h{}; for being cryptic.

I did appreciate the combining the extraction of the result into the operation itself, but it's less modular (i.e. complicates things if you want to work with the resulting set).

In reply to Re^4: Subtracting Lists by ikegami
in thread Subtracting Lists by JimSki

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