Goal:
  
  A : Type
  B : Type
  k : nat
  Q : nat → A → Type
  R : nat → A → B → Type
  l : list A
  l' : list B
  n : nat
  x : A
  y : B
  xs : list A
  ys : list B
  r : R n x y
  rs : All2i R (S n) xs ys
  ih : Alli Q (S n) xs
       → All2i (λ (i : nat) (x : A) (y : B), Q i x × R i x y) (S n) xs ys
  q1 : sigma (λ _ : nat, list A)
  q : Alli Q (pr1 q1) (pr2 q1)
  ============================
   ({| pr1 := n; pr2 := x :: xs |} = q1
    → let H := CoreTactics.block in
      let H0 := CoreTactics.block in
      All2i (λ (i : nat) (x : A) (y : B), Q i x × R i x y) n 
        (x :: xs) (y :: ys))


DEBUG:Going to execute:
hnf in id
Level 4: evaluation returns
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
where
id' := q
of type
intropattern
id := q1
of type
constr of type tacvalue

Level 3: evaluation returns
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
where
id' := q
of type
intropattern
id := q1
of type
constr of type tacvalue