consider b being Element of L such that
A3: B = (StoneH L) . b by Th13;
consider a being Element of L such that
A4: A = (StoneH L) . a by Th13;
A /\ B = (StoneH L) . (a "/\" b) by A4, A3, Th15;
hence A /\ B is Element of StoneS L by Th13; :: thesis: verum