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

theorem
  for x being Real holds x in ].u,+infty.[ iff u < x
proof
  let x be Real;
  x in REAL by XREAL_0:def 1;
  then x < +infty by XXREAL_0:9;
  hence thesis by Th4;
end;
