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
  q : Alli Q n (x :: xs)
  q0 := {| pr1 := {| pr1 := n; pr2 := x :: xs |}; pr2 := q |}
     : sigma
         (λ index : sigma (λ _ : nat, list A), Alli Q (pr1 index) (pr2 index))
  q1 := {| pr1 := n; pr2 := x :: xs |} : sigma (λ _ : nat, list A)
  q2 := q
     : (λ index : sigma (λ _ : nat, list A), Alli Q (pr1 index) (pr2 index))
         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:
match goal with
           | id'1:=?t:_
             |- _ =>
                 generalize (eq_refl : t = id'1); clearbody id'2 id'1; clear
                  id' id; try unfold signature in id'2; 
                  hnf in id'2; simpl in id'2; rename id'2 into id;
                  cont id id id'1 t
           end
Level 1: Backtrace:

No head constant to reduce.