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

theorem Th86:
  0 < y & y <> +infty implies -infty / y = -infty
proof
  assume that
A1: 0<y and
A2: y<>+infty;
  y" <> 0 by A1,A2,Th82;
  hence thesis by A1,Def5;
end;
