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 ].-infty,u.] iff x <= u
proof
  let x be Real;
  x in REAL by XREAL_0:def 1;
  then -infty < x by XXREAL_0:12;
  hence thesis by Th2;
end;
