added VernacBacktrack (new backtracking command dedicated to interfaces like PG, coqied or pcoq) in order to get rid of a pattern matching not exhaustive.
added VernacBacktrack (new backtracking command dedicated to
interfaces like PG, coqied or pcoq) in order to get rid of a pattern
matching not exhaustive.


git-svn-id: a49b465eaba45b67e2e1e4e0b2c256a71f59fecb@7038 85f007b7-540e-0410-9357-904b9bb8a0f7

SamB/coq