I also reverted 7952bca4 since there is no need for atomic to be a
boolean predicate anymore. Moreover, I introduced a hint database
fsaV for solving side-conditions related to FSAs, in particular,
side-conditions related to expressions being atomic.

We reify to a representation of expressions that includes an explicit
constructor for closed terms. Substitution can then be implemented as
the identify, which enables us to perform it using computation.

This reverts commit 6076bbc6.

The intropattern {H} also meant clear (both in ssreflect, and the logic
part of the introduction pattern).