Goal:
  
  A : Type
  P : nat → A → A → Type
  Q : nat → A → A → Type
  i : nat
  hd : A
  tl : list A
  tl' : list A
  X : OnOne2i P (S i) tl tl'
  IHX : (∀ (i : nat) (x y : A), P i x y → Q i x y) → OnOne2i Q (S i) tl tl'
  X0 : ∀ (i : nat) (x y : A), P i x y → Q i x y
  ============================
   (OnOne2i Q (S i) tl tl')


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