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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.

This prevent the assumption from being dragged into lemmas that do not even need it

(missed practically everything in the previous commit)

Also, make our redefinition of done more robust under different
orders of Importing modules.

Since Coq 8.4 did not backtrack on eauto premises, we used to ensure
that hints like

  Hint Extern 0 (?x ≡{_}≡ ?y) => reflexivity.

were not used for goals involving evars by writing ?x ≡{_}≡ ?y instead
of _ ≡{_}≡ _.

This seems to be a legacy issue that no longer applies to Coq 8.5, so
I have removed these restrictions making these hints thus more powerful.

Also do some minor clean up.