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

theorem Th24:
  [.p,q.[ c= [.p,q.]
proof
  let s;
  assume
A1: s in [.p,q.[;
  then
A2: p <= s by Th3;
  s < q by A1,Th3;
  hence thesis by A2,Th1;
end;
