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- The direction of big_sepS_later and big_sepM_later is now like later_sep.
- Do not use generated variables in the proofs.

It is doing much more than just dealing with ∈, it solves all kinds
of goals involving set operations (including ≡ and ⊆).

Also, specialize the big ops to gmap and gset because that is all that
we are using. For the big ops on sets this also means we can use Leibniz
equality on sets.

The singleton maps notation is now also more consistent with the
insert <[_ := _]> _ notation for maps.

We now have:

  Π★{map Q } ...
  Π★{set Q } ...

to differentiate between sets and maps.

With nicely overloaded notations for sets and maps.