reserve x,y,z,r,s for ExtReal;
reserve A,B for ext-real-membered set;

theorem Th39:
  sup {} = -infty
proof
A1: for x being UpperBound of {} holds -infty <= x by XXREAL_0:5;
  -infty is UpperBound of {} by Th37;
  hence thesis by A1,Def3;
end;
