Goal:
  
  A : Type
  B : Type
  P : A → Type
  Q : A → B → Type
  x : A
  l : list A
  p : P x
  H1 : All P l
  H2 : ∀ (x : A) (y : B), In y [] → P x → Q x y
  Hl1 : nat
  Hl : 0 = Hl1
  ============================
   (S #|l| = Hl1
    → let H := CoreTactics.block in
      (∀ l' : list B,
         (∀ (x : A) (y : B), In y l' → P x → Q x y)
         → #|l'| = #|l| → All2 Q l l')
      → let H0 := CoreTactics.block in All2 Q (x :: l) [])


DEBUG:Going to execute:
hnf in id;
            match type of id with
            | sigma _ =>
                let xn := fresh id "'" in
                destruct id as [xn id]; DepElim.decompose_exists xn id;
                 cbv beta iota delta [TODO projection TODO projection]
                  in id, id'; DepElim.decompose_exists id id'
            | _ =>
                cbv beta iota delta [TODO projection TODO projection]
                 in id, id'
            end