let p, q, r, s be ExtReal; :: thesis: ( q < s & r < s implies not ].r,s.[ c= [.p,q.[ )
assume that
A1: q < s and
A2: r < s ; :: thesis: not ].r,s.[ c= [.p,q.[

per cases ( r <= q or q < r ) ;

suppose A3: r <= q ; :: thesis: not ].r,s.[ c= [.p,q.[

consider t being ExtReal such that
A4: q < t and
A5: t < s by A1, XREAL_1:227;
take t ; :: according to MEMBERED:def 8 :: thesis: ( t in ].r,s.[ & not t in [.p,q.[ )
r < t by A3, A4, XXREAL_0:2;
hence t in ].r,s.[ by A5, Th4; :: thesis: not t in [.p,q.[
thus not t in [.p,q.[ by A4, Th3; :: thesis: verum

end;

suppose A6: q < r ; :: thesis: not ].r,s.[ c= [.p,q.[

consider t being ExtReal such that
A7: r < t and
A8: t < s by A2, XREAL_1:227;
take t ; :: according to MEMBERED:def 8 :: thesis: ( t in ].r,s.[ & not t in [.p,q.[ )
thus t in ].r,s.[ by A7, A8, Th4; :: thesis: not t in [.p,q.[
q < t by A6, A7, XXREAL_0:2;
hence not t in [.p,q.[ by Th3; :: thesis: verum

end;

end;