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

theorem Th78:
  x <> -infty & x <> +infty & x <> 0 implies x / x = 1
proof
  assume x <> -infty & x <> +infty;
  then x in REAL by XXREAL_0:14;
  then reconsider a=x as Real;
  assume x <> 0;
  then a/a =1 by XCMPLX_1:60;
  hence thesis;
end;
