reserve x,y,w,z for ExtReal,
  a for Real;

theorem Th19:
  |.+infty.| = +infty & |.-infty.| = +infty
proof
  thus |.+infty.| = +infty by Def1;
  --infty = +infty by XXREAL_3:23;
  hence thesis by Def1;
end;
