Goal:
  
  A : Type
  P : nat → A → Type
  f : nat → A → A
  n : nat
  H : ∀ (n : nat) (x : A), P n x → f n x = x
  X : A → Type
  ============================
   ([] = [])


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