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

theorem
  [.-infty,q.] \ ].-infty,s.] = {-infty} \/ ].s,q.]
proof
A1: -infty <= s by XXREAL_0:5;
  -infty <= q by XXREAL_0:5;
  hence thesis by A1,Th324;
end;
