Given an term t and a theorem A |- t1 = t2, which is an equation between
boolean terms, CONJ_DISCH returns A - {t} |- (t /\ t1) = (t /\ t2), i.e.
conjoins t to both sides of the equation, removing t from the assumptions
if it was there.
A |- t1 = t2
------------------------------ CONJ_DISCH "t"
A - {t} |- t /\ t1 = t /\ t2