* Put level of the triple at 20, so we can write things like
  ▷ {{ P }} e @ E {{ Φ }} without parentheses.
* Use high levels for P, e and Φ.
* Allow @ E to be omitted in case E = ⊤.

This fix removes superflous white space in pretty printing. For example
✓{S n} x was pretty printed incorrectly as ✓{(S n)} x.

Still need to use it everywhere.

turns out we can import with shorter names (and we actually already do that frequently in "From Coq Import")

This avoids ambiguity with P and Q that we were using before for both
uPreds/iProps and indexed uPreds/iProps.

- 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 ⊆).

simplify_equality        => simplify_eq
simplify_equality'       => simplify_eq/=
simplify_map_equality    => simplify_map_eq
simplify_map_equality'   => simplify_map_eq/=
simplify_option_equality => simplify_option_eq
simplify_list_equality   => simplify_list_eq
f_equal'                 => f_equal/=

The /= suffixes (meaning: do simpl) are inspired by ssreflect.

The tactic injection H as H is doing exactly that.