
theorem
  for x,y being ExtReal, a,b being Real
   st x = a & y = b holds x - y = a - b
proof
  let x,y be ExtReal, a,b be Real;
  assume
A1: x = a;
  assume y = b;
  then -y = -b by XXREAL_3:def 3;
  hence thesis by A1,XXREAL_3:def 2;
end;
