induction' as with a as ih generalizing bs
α✝ : Type u
inst✝² : DecidableEq α✝
β✝ : Type v
inst✝¹ : DecidableEq β✝
m : Type ?u.156032 → Type ?u.156034
α : Type u_2
β : Type u_1
inst✝ : Monad m
bs : List β
f : α → Option β
as : List α
⊢ mapM' f as = some bs → bs.length = as.length
case nil
α✝ : Type u
inst✝² : DecidableEq α✝
β✝ : Type v
inst✝¹ : DecidableEq β✝
m : Type ?u.156032 → Type ?u.156034
α : Type u_2
β : Type u_1
inst✝ : Monad m
f : α → Option β
bs : List β
⊢ mapM' f [] = some bs → bs.length = [].length

case cons
α✝ : Type u
inst✝² : DecidableEq α✝
β✝ : Type v
inst✝¹ : DecidableEq β✝
m : Type ?u.156032 → Type ?u.156034
α : Type u_2
β : Type u_1
inst✝ : Monad m
f : α → Option β
a : α
as : List α
ih : ∀ {bs : List β}, mapM' f as = some bs → bs.length = as.length
bs : List β
⊢ mapM' f (a :: as) = some bs → bs.length = (a :: as).length