reserve E for non empty set;
reserve a for Element of E;
reserve A, B for Subset of E;
reserve Y for set;
reserve p for FinSequence;
reserve e, e1, e2 for Singleton of E;

theorem Th13:
  A /\ B misses A /\ B`
proof
  A /\ B misses A \ B by XBOOLE_1:89;
  hence thesis by SUBSET_1:13;
end;
