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

theorem Th28:
  |.x.| = +infty implies x = +infty or x = -infty
proof
A1: |.x.| = x or -|.x.| = --x by Def1;
  assume |.x.| = +infty;
  hence thesis by A1,XXREAL_3:5;
end;
