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

theorem Th14:
  x <> -infty implies -infty - x = -infty & x - -infty = +infty
proof
  assume
A1: x <> -infty;
  now
    assume -x = +infty;
    then --x = -infty by Def3;
    hence contradiction by A1;
  end;
  hence -infty - x = -infty by Def2;
  thus thesis by A1,Def2,Th5;
end;
