let t be ext-real number ; :: thesis: for s being real number holds ].-infty,t.] \ ].-infty,s.[ = [.s,t.]
let s be real number ; :: thesis: ].-infty,t.] \ ].-infty,s.[ = [.s,t.]
s in REAL by XREAL_0:def 1;
hence ].-infty,t.] \ ].-infty,s.[ = [.s,t.] by Th188, XXREAL_0:12; :: thesis: verum