reserve x for set,
  p,q,r,s,t,u for ExtReal,
  g for Real,
  a for Element of ExtREAL;

theorem
  q <= s & r < s implies not ].r,s.] c= [.p,q.[
proof
  assume that
A1: q <= s and
A2: r < s;
  take t=s;
  thus t in ].r,s.] by A2,Th2;
  thus thesis by A1,Th3;
end;
