Goal:
  
  A : Type
  P : nat → A → Type
  f : A → A
  n : nat
  hd : A
  tl : list A
  p : P n hd
  X : Alli P (S n) tl
  IHX : (∀ (n : nat) (x : A), P n x → f x = x) → map f tl = tl
  H : ∀ (n : nat) (x : A), P n x → f x = x
  ============================
   (f hd :: map f tl = hd :: tl)


DEBUG:Going to execute:
assert X2
Level 1: evaluation returns
assert X2; [ exact (id0 id1) | clear id0 ]
where
id1 := H
of type
intropattern
id0 := IHX
of type
intropattern
flags := 
of type
tauto_flags
X2 := map f tl = tl
of type
constr of type tacvalue