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

theorem
  a+b <= c-d implies a+d <= c-b
proof
  assume a+b <= c-d;
  then a+b+d <= c-d+d by Lm5;
  then a+b+d-b <= c-b by Th9;
  hence thesis;
end;
