rintro b _ bs hbs rfl
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
⊢ ∀ (x : β), f a = some x → ∀ (x_1 : List β), mapM' f as = some x_1 → x :: x_1 = bs → bs.length = as.length.succ
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
b : β
a✝ : f a = some b
bs : List β
hbs : mapM' f as = some bs
⊢ (b :: bs).length = as.length.succ