reserve X for set;

theorem Th19:
  for A,B being Subset of X ex F being sequence of bool X st
  rng F = {A,B} & F.0 = A & for n being Nat st 0 < n holds F.n = B
proof
  let A,B be Subset of X;
  take A followed_by B;
  thus thesis by FUNCT_7:119,120,126;
end;
