
theorem Th2:
  for L being RelStr, x being Element of L, y being Element of L
  opp holds (x <= ~y iff x~ >= y) & (x >= ~y iff x~ <= y)
proof
  let L be RelStr, x be Element of L, y be Element of L opp;
  ~(x~) = x~;
  hence thesis by Th1;
end;
