reserve x, x1, x2, y, z, X9 for set,
  X, Y for finite set,
  n, k, m for Nat,
  f for Function;
reserve F,Ch for Function;

theorem Th25:
 for y being object holds
  Intersection(F,{},y)=union rng F
proof let y be object;
  F|({}"{y} qua set)=({}"{y}-->union rng F);
  hence thesis by Th23;
end;
