Moves a paired existential quantification outwards through a conjunction.
DESCRIPTION
When applied to a term of the form (?p. t) /\ (?p. u), where no
variables in p are free in either t or u, AND_PEXISTS_CONV returns
the theorem:
|- (?p. t) /\ (?p. u) = (?p. t /\ u)
FAILURE
AND_PEXISTS_CONV fails if it is applied to a term not of the form
(?p. t) /\ (?p. u), or if it is applied to a term (?p. t) /\ (?p. u)
in which variables from p are free in either t or u.