Goal:
  
  X : Type
  x : X
  n : nat
  l1 : list X
  l2 : list X
  l : n < #|l1|
  H : nth_error (l1 ++ l2) n = Some x
  ============================
   (n < #|l1| ∧ nth_error l1 n = Some x
    ∨ n ≥ #|l1| ∧ nth_error l2 (n - #|l1|) = Some x)


DEBUG:Going to execute:
rewrite nth_error_app1 in   H