Goal:
  
  A : Type
  B : Type
  R : A → B → Type
  l : list A
  a : B
  r1 : list B
  r2 : list B
  h : All2 R l (a :: r1 ++ r2)
  h0 := {| pr1 := {| pr1 := l; pr2 := a :: r1 ++ r2 |}; pr2 := h |}
     : sigma
         (λ index : sigma (λ _ : list A, list B),
            All2 R (pr1 index) (pr2 index))
  h1 := {| pr1 := l; pr2 := a :: r1 ++ r2 |} : sigma (λ _ : list A, list B)
  h2 := h
     : (λ index : sigma (λ _ : list A, list B),
          All2 R (pr1 index) (pr2 index)) h1
  ============================
   (let H := CoreTactics.block in
    (∀ (l : list A) (r2 : list B),
       All2 R l (r1 ++ r2)
       → ∑ l1 l2 : list A, l = l1 ++ l2 × All2 R l1 r1 × All2 R l2 r2)
    → let H0 := CoreTactics.block in
      ∑ l1 l2 : list A, l = l1 ++ l2 × All2 R l1 (a :: r1) × All2 R l2 r2)


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.