Goal:
  
  A : Type
  B : Type
  P : A → B → Type
  t' : B
  x : A
  l : list A
  l' : list B
  r : P x t'
  Hall : All2 P l l'
  IHHall : ∀ n : nat,
             nth_error l' n = Some t'
             → ∑ t : A, nth_error l n = Some t × P t t'
  ============================
   (P x t')


DEBUG:Going to execute:
assert X2
Level 1: evaluation returns
assert X2; [ exact (id0 id1) | clear id0 ]
where
id1 := x
of type
intropattern
id0 := P
of type
intropattern
flags := 
of type
tauto_flags
X2 := B → Type
of type
constr of type tacvalue