let t be ExtReal; :: thesis: for s being Real holds [.-infty,t.[ \ [.-infty,s.[ = [.s,t.[
let s be Real; :: thesis: [.-infty,t.[ \ [.-infty,s.[ = [.s,t.[
s in REAL by XREAL_0:def 1;
hence [.-infty,t.[ \ [.-infty,s.[ = [.s,t.[ by Th185, XXREAL_0:12; :: thesis: verum