theorem
  (A /\ B)? = (A?) /\ (B?)
proof
  thus (A /\ B)? = {<%>E} \/ (A /\ B) by Th76
    .= ({<%>E} \/ A) /\ ({<%>E} \/ B) by XBOOLE_1:24
    .= (A?) /\ ({<%>E} \/ B) by Th76
    .= (A?) /\ (B?) by Th76;
end;
