#89 Make e! and m! take a block to filter permutations right away

[Martin Büttner]

This one isn't really for golfing, as it would only save one byte. But I think it would be really convenient (and allow one to write more memory-efficient programs) if e! and m! took an optional block to filter the permutations right away. If you only want a small part of the permutations then filtering while they are generated would save a lot of memory.

Basically, these should be equivalent:

[1 2 2 3]e!{...},
[1 2 2 3]{...}e!

I know, I know, inconsistent operator parity and stuff, but , and $ are also unary operators which take an optional block (and then still operator on the array underneath).

[Martin Büttner]

I guess m* would also be a good candidate for this feature, but it's not unary to begin with.