Goal:
  
  X : Type
  x : X
  n : nat
  l1 : list X
  l2 : list X
  l : #|l1| ≤ n
  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:
<coq-core.plugins.ssreflect::ssrrewrite@0> $1 $2

Goals:
    X : Type  x : X  n : nat  l1 : list X  l2 : list X  l : #|l1| ≤ n
  ============================
   (#|l1| ≤ n)

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


DEBUG:Going to execute:
eauto
DEBUG LTAC Evaluated term: nth_error_app2
DEBUG LTAC Evaluated term: nth_error_app2

Goals:
    X : Type  x : X  n : nat  l1 : list X  l2 : list X  l : #|l1| ≤ n
  ============================
   (#|l1| ≤ n)

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


DEBUG:Going to execute:
<coq-core.plugins.ltac::eauto@0> $1 $2 $3