Little reorganization of generalize tactics code w/o semantic changes.
Also removing trailing spaces.
Renaming new_induct -> induction; new_destruct -> destruct.
Simplification and improvement of "subst x" in such a way that it
works in the presence of local definitions referring to x and
dependent in other hyps or concl.
Isolating a function "make_abstraction", new name of "letin_abstract",
which compute an abstraction of the goal over a term or a pattern.
Typo reference manual
Avoid "revert" to retype-check the goal, and move it to a "new" tactic.