consider b being Element of L such that
A3: B = (StoneH L) . b by Th15;
consider a being Element of L such that
A4: A = (StoneH L) . a by Th15;
A /\ B = (StoneH L) . (a "/\" b) by A4, A3, Th17;
hence A /\ B is Element of StoneS L by Th15; :: thesis: verum