Goal:
    A : Type  B : Type  P : nat → A → B → Type  n : nat  x : B  n0 : nat
  ============================
   (nth_error [] n = Some x
    → ∑ c : A, nth_error [] n = Some c × P (n0 + n)%nat c x)


DEBUG:Going to execute:
<coq-core.plugins.ssreflect::ssrtclintros@0> arg