let s be real number ; :: thesis: [.-infty ,s.[ = {-infty } \/ ].-infty ,s.[
s in REAL by XREAL_0:def 1;
then -infty < s by XXREAL_0:12;
hence [.-infty ,s.[ = {-infty } \/ ].-infty ,s.[ by Th131; :: thesis: verum