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

theorem Th13:
  a <= b & c <= d implies a-d <= b-c
proof
  assume
A1: a <= b;
  assume c <= d;
  then -d <= -c by Lm14;
  then a+-d <= b+-c by A1,Lm6;
  hence thesis;
end;
