reserve fi,psi for Ordinal-Sequence,
  A,A1,B,C,D for Ordinal,
  X,Y for set,
  x,y for object;

theorem Th8:
  A <> {} implies {} in A
proof
A1: {} c= A;
  assume A <> {};
  then {} c< A by A1;
  hence thesis by ORDINAL1:11;
end;
