merged

Prototype introiff option for find_theorems. This feature was suggested by Jeremy Avigad / Tobias Nipkow. It adds an introiff keyword for find_theorems that returns, in addition to the usual results for intro, any theorems of the form ([| ... |] ==> (P = Q)) where either P or Q matches the conclusions of the current goal. Such theorems can be made introduction rules with [THEN iffDx]. The current patch is for evaluation only. It assumes an (op = : 'a -> 'a -> bool) operator, which is specific to HOL. It is not clear how this should be generalised.