CHANGED_TAC : (tactic -> tactic)
STRUCTURE
SYNOPSIS
Makes a tactic fail if it has no effect.
DESCRIPTION
When applied to a tactic T, the tactical CHANGED_TAC gives a new tactic which is the same as T if that has any effect, and otherwise fails.
FAILURE
The application of CHANGED_TAC to a tactic never fails. The resulting tactic fails if the basic tactic either fails or has no effect.
SEEALSO
HOL  Kananaskis-14