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

theorem Th50:
  r <= s & [.r,s.] c= [.p,q.] implies p <= r & s <= q
proof
  assume
A1: r <= s;
  then
A2: r in [.r,s.] by Th1;
  s in [.r,s.] by A1,Th1;
  hence thesis by A2,Th1;
end;
