Goal:
  
  A : Type
  l3 : list A
  R1 : A → A → Type
  R2 : A → A → Type
  R3 : A → A → Type
  hd : A
  tl : list A
  tl' : list A
  o : OnOne2 R1 tl tl'
  IHo : ∀ l3 : list A,
          All2 R2 tl l3
          → (∀ x y : A, R2 x y → R3 x y)
            → (∀ x y z : A, R1 x y → R2 x z → R3 y z) → All2 R3 tl' l3
  H : All2 R2 (hd :: tl) l3
  H0 := {| pr1 := {| pr1 := hd :: tl; pr2 := l3 |}; pr2 := H |}
     : sigma
         (λ index : sigma (λ _ : list A, list A),
            All2 R2 (pr1 index) (pr2 index))
  H1 := {| pr1 := hd :: tl; pr2 := l3 |} : sigma (λ _ : list A, list A)
  H2 := H
     : (λ index : sigma (λ _ : list A, list A),
          All2 R2 (pr1 index) (pr2 index)) H1
  ============================
   (let H := CoreTactics.block in
    let H0 := CoreTactics.block in
    (∀ x y : A, R2 x y → R3 x y)
    → (∀ x y z : A, R1 x y → R2 x z → R3 y z) → All2 R3 (hd :: tl') l3)


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.