simp only [Nat.sub_sub]
m : Type u → Type v
inst✝ : Monad m
α : Type u
f : Nat → α → m α
init : α
n k : Nat
h_ind : k ≤ n → foldM.loop f (n + 1) k = foldM.loop (f ∘ succ) n k
hk : succ k ≤ n
⊢ n + 1 - k - 1 = succ n - succ k
no goals