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


DEBUG:Going to execute:
<coq-core.plugins.ssreflect::ssrrewrite@0> $1 $2