let r, s, t be ExtReal; :: thesis: ( r < s & s <= t implies ].r,s.] \/ [.s,t.] = ].r,t.] )
assume that
A1: r < s and
A2: s <= t ; :: thesis: ].r,s.] \/ [.s,t.] = ].r,t.]
let p be ExtReal; :: according to MEMBERED:def 14 :: thesis: ( ( not p in ].r,s.] \/ [.s,t.] or p in ].r,t.] ) & ( not p in ].r,t.] or p in ].r,s.] \/ [.s,t.] ) )
thus ( p in ].r,s.] \/ [.s,t.] implies p in ].r,t.] ) :: thesis: ( not p in ].r,t.] or p in ].r,s.] \/ [.s,t.] )

proof

assume p in ].r,s.] \/ [.s,t.] ; :: thesis: p in ].r,t.]
then ( p in ].r,s.] or p in [.s,t.] ) by XBOOLE_0:def 3;
then A3: ( ( r < p & p <= s ) or ( s <= p & p <= t ) ) by Th1, Th2;
then A4: r < p by A1, XXREAL_0:2;
p <= t by A2, A3, XXREAL_0:2;
hence p in ].r,t.] by A4, Th2; :: thesis: verum

end;

assume p in ].r,t.] ; :: thesis: p in ].r,s.] \/ [.s,t.]
then ( ( r < p & p <= s ) or ( s <= p & p <= t ) ) by Th2;
then ( p in ].r,s.] or p in [.s,t.] ) by Th1, Th2;
hence p in ].r,s.] \/ [.s,t.] by XBOOLE_0:def 3; :: thesis: verum