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
  e1 = e2 or e1 misses e2
proof
  e1 /\ e2 c= e1 by XBOOLE_1:17;
  then e1 /\ e2 = {} or e1 /\ e2 = e1 by Th1;
  then e1 c= e2 or e1 /\ e2 = {} by XBOOLE_1:17;
  hence thesis by Th1;
end;
