trace message from a different task
def foo : Π {α : Type u} {n : ℕ}, array (0 + n) α → array n α :=
λ {α : Type u} {n : ℕ}, ite (n = 0) (λ (v : array (0 + n) α), cast _ v) (λ (v : array (0 + n) α), cast _ v)