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This one (previously solve_elem_of) was hardly used. The tactic that uses
naive_solver (previously esolve_elem_of, now solve_elem_of) has been
extended with flags to say which hypotheses should be cleared/kept.

This is a workarround, see: https://github.com/math-comp/math-comp/issues/18

Sadly, timelessness of many connectives is still proved in the model.

Now we use a specific notation (⊑) to whose arguments we interpret
in uPred_scope. This makes notations much more concise.