reserve a,b,c for set;

theorem Th5:
  for T being non empty TopStruct for x being Point of T holds Chi(
  x,T) c= Chi(T)
proof
  let T be non empty TopStruct;
  set X = the set of all Chi(x,T) where x is Point of T;
  let x be Point of T;
A1: Chi(x,T) in X;
  Chi(T) = union X by Th4;
  hence thesis by A1,ZFMISC_1:74;
end;
