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

theorem
  A*^B = {} implies A = {} or B = {}
proof
  assume that
A1: A*^B = {} and
A2: A <> {} and
A3: B <> {};
  {} c= A;
  then {} c< A by A2;
  hence contradiction by A1,A3,ORDINAL1:11,ORDINAL2:40;
end;
