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

theorem Th314:
  r <= p & p <= q implies [.r,q.[ \ {p} = [.r,p.[ \/ ].p,q.[
proof
  [.p,p.] = {p} by Th17;
  hence thesis by Th310;
end;
