Goal:
  
  A : Type
  B : Type
  C : Type
  R : A → C → Type
  g : A → B → C
  x : A
  xs : list A
  ih : ∀ l' : list B,
         #|xs| = #|l'|
         → All2 R xs (map2 g xs l')
           → All2 (λ (x : A) (y : B), R x (g x y)) xs l'
  y : B
  ys : list B
  eq : #|xs| = #|ys|
  z : All2 R (x :: xs) (g x y :: map2 g xs ys)
  z0 := {|
          pr1 := {| pr1 := x :: xs; pr2 := g x y :: map2 g xs ys |}; pr2 := z
        |}
     : sigma
         (λ index : sigma (λ _ : list A, list C),
            All2 R (pr1 index) (pr2 index))
  ============================
   (let H := CoreTactics.block in
    let H0 := CoreTactics.block in
    All2 (λ (x : A) (y : B), R x (g x y)) (x :: xs) (y :: ys))


DEBUG:Going to execute:
lazymatch goal with
           | id':=?v:_
             |- context [ id ] =>
                 generalize (eq_refl : v = id'); clearbody id'; simpl in id';
                  cont id id' id v
           | id':=?v:_
             |- _ =>
                 let id'1 := fresh id' in
                 let id'2 := fresh id' in
                 set (id'2 := pr2 id'); set (id'1 := pr1 id') in id'2;
                  hnf in (value of id'1), (value of id'2);
                  try red in (type of id'2);
                  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
           end