reserve a,b,c,d for Real;
reserve r,s for Real;

theorem
  a-b <= c-d implies d-c <= b-a
proof
  assume a-b <= c-d;
  then a-b+d <= c by Lm17;
  then a+d-b <= c;
  then a+d <= c+b by Lm19;
  hence thesis by Lm20;
end;
