Drule.iffLR

A call to iffLR th, where th has the form

   A |- !x1 .. xn. p1 /\ .. /\ pm ==> !y... q1 /\ .. ==> ... ==> (l <=> r)

returns the left-to-right implication

   A |- !x1 .. xn. p1 /\ .. /\ pm ==> !y... q1 /\ .. ==> ... ==> l ==> r

The universal variables and various antecedents are said to “guard” the if-and-only-if conclusion l <=> r in this situation. They may be nested abitrarily deep, or not present at all. They are restored after a call to EQ_IMP_RULE is made.