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


DEBUG:Going to execute:
constructor 1
Level 3: evaluation returns
<coq-core.plugins.tauto::is_unit_or_eq@0>
where
tauto_flags := 
of type
tauto_flags
X1 := [] = []
of type
constr of type tacvalue