Goal:
  
  A : Type
  P : nat → A → Type
  Q : A → Type
  n : nat
  hd : A
  tl : list A
  p : P n hd
  X : Alli P (S n) tl
  IHX : All Q tl → Alli (λ (k : nat) (x : A), P k x × Q x) (S n) tl
  X0 : All Q (hd :: tl)
  x : A
  l : list A
  X1 : Q hd
  X2 : All Q tl
  H0 : x = hd
  H1 : l = tl
  ============================
   (P n hd × Q hd)


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