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

theorem Th313:
  r <= p & p <= q implies ].r,q.[ \ {p} = ].r,p.[ \/ ].p,q.[
proof
  [.p,p.] = {p} by Th17;
  hence thesis by Th309;
end;
