theorem Th66:
  r <=' s & s <=' r implies r = s
proof
  given x such that
A1: s = r+x;
  given y such that
A2: r = s+y;
  r+{} = r by Th50
    .= r+(x+y) by A1,A2,Th51;
  then x = {} by Th62,Th63;
  hence thesis by A1,Th50;
end;
