take 0 ; :: according to XXREAL_2:def 10 :: thesis: 0 is UpperBound of {-infty}
let x be ExtReal; :: according to XXREAL_2:def 1 :: thesis: ( x in {-infty} implies x <= 0 )
assume x in {-infty} ; :: thesis: x <= 0
hence x <= 0 by TARSKI:def 1; :: thesis: verum