reserve x,y,z,w for ExtReal,
  r for Real;
reserve f,g for ExtReal;

theorem Th37:
  x <= y & z <= w implies x - w <= y - z
proof
  assume that
A1: x <= y and
A2: z <= w;
  -w <= -z by A2,Lm15;
  hence thesis by A1,Th36;
end;
