let s be real number ; :: thesis: [.-infty ,s.] \ ].-infty ,s.] = {-infty }
s in REAL by XREAL_0:def 1;
then -infty < s by XXREAL_0:12;
hence [.-infty ,s.] \ ].-infty ,s.] = {-infty } by Th403; :: thesis: verum