In particular, make sure we always try eassumption before reflexivity.

The old name didn't make much sense. Also now we can have
pure_False too.

That range includes tabs and new lines.

Thanks Morten for spotting this problem.

This way we can use set_solver to solve goals involving ∈.

This has bothered me repeatedly in proofs, now I finally got around to fix it at the source

This reverts commit 2a7755fe because
it is no longer needed after Matthieu Sozeau reverted this change in
Coq 8.6. See also the discussion:

  [Coq-Club] Coq 8.6 typeclasses behavior change

at 11/16/2016 02:14 PM.

We need instances like EqDecision and Countable for it. We could
redeclare those instead, though.