theorem Th4:
  X is trivial & X \/ {y} is non trivial implies
   ex x being object st X = {x}
proof
  assume that
A1: X is trivial and
A2: X \/ {y} is non trivial;
  X is empty or ex x being object st X = {x} by A1,ZFMISC_1:131;
  hence thesis by A2;
end;
