Goal:
  
  A : Type
  B : Type
  C : Type
  R : nat → A → C → Type
  g : A → B → C
  x : A
  xs : list A
  ih : ∀ (l' : list B) (k : nat),
         All2i (λ (i : nat) (x : A) (y : B), R i x (g x y)) k xs l'
         → All2i R k xs (map2 g xs l')
  y : B
  ys : list B
  k : nat
  z : All2i (λ (i : nat) (x : A) (y : B), R i x (g x y)) k 
        (x :: xs) (y :: ys)
  z0 := {|
          pr1 := {| pr1 := k; pr2 := {| pr1 := x :: xs; pr2 := y :: ys |} |};
          pr2 := z
        |}
     : sigma
         (λ index : sigma (λ _ : nat, sigma (λ _ : list A, list B)),
            All2i (λ (i : nat) (x : A) (y : B), R i x (g x y)) 
              (pr1 index) (pr1 (pr2 index)) (pr2 (pr2 index)))
  z1 := {| pr1 := k; pr2 := {| pr1 := x :: xs; pr2 := y :: ys |} |}
     : sigma (λ _ : nat, sigma (λ _ : list A, list B))
  z2 := z
     : (λ index : sigma (λ _ : nat, sigma (λ _ : list A, list B)),
          All2i (λ (i : nat) (x : A) (y : B), R i x (g x y)) 
            (pr1 index) (pr1 (pr2 index)) (pr2 (pr2 index))) z1
  ============================
   (let H := CoreTactics.block in
    let H0 := CoreTactics.block in R k x (g x y))


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.