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 Th6:
  ex a st a in E & e = {a}
proof
  set x = the Element of e;
  {x} = e by Th1;
  hence thesis;
end;
