exact pure_bind ..
case h
α✝ : Type u
m : Type u → Type v
inst✝¹ : Monad m
inst✝ : LawfulMonad m
α : Type u
x : Array α
n : Nat
f : α → m α
h : n < size x
a : α
⊢ (do
      let a ←
        pure
            { val := set x { val := n, isLt := h } a,
              property := (_ : size (set x { val := n, isLt := h } a) = size x) }
      pure a.val) =
    pure (set x { val := n, isLt := h } a)
no goals