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

theorem
  a+b <= c+d iff a-c <= d-b
proof
  thus a+b <= c+d implies a-c <= d-b by Lm20;
  assume a-c <= d-b;
  then a-c+b <= d-b+b by Lm5;
  then a-c+b+c <= d+c by Lm5;
  hence thesis;
end;
