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