Fixpoint Pos_log2ceil_plus1 (p : positive) : positive :=
  match p with
  | (p'~1)%positive => Pos.succ (Pos.succ (Pos_log2floor_plus1 p'))
  | (p'~0)%positive => Pos.succ (Pos_log2ceil_plus1 p')
  | 1%positive      => 1
  end.

Lemma Pos_log2ceil_plus1_spec : forall (p : positive), (Pos.pow 2 (Pos_log2ceil_plus1 p) < 4 * p <= 2 * Pos.pow 2 (Pos_log2ceil_plus1 p))%positive.