Goal:
  
  A : Type
  B : Type
  P : nat → A → B → Type
  n : nat
  x : B
  n0 : nat
  H : None = Some x
  ============================
   (∑ c : A, nth_error [] n = Some c × P (n0 + n)%nat c x)


DEBUG:Going to execute:
apply : eq_sym