CHANGED_TAC : (tactic -> tactic)

SYNOPSIS
Makes a tactic fail if it has no effect.

DESCRIBE
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  TRY,   VALID

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