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

theorem Th84:
  y < 0 & -infty <> y implies -infty / y = +infty
proof
  assume that
A1: y<0 and
A2: -infty<>y;
  y" <> 0 by A1,A2,Th82;
  hence thesis by A1,Def5;
end;
