theorem
  x*y = 0.SF implies x = 0.SF or y = 0.SF
proof
  now
    assume that
A1: x*y = 0.SF and
A2: x <> 0.SF;
    x*y = x*(0.SF) by A1;
    hence y = 0.SF by A2,Th8;
  end;
  hence thesis;
end;
