Goal:
  
  A : Type
  B : Type
  C : Type
  P : nat → A → B → Type
  Q : B → C → Type
  R : B → C → Type
  l'' : list C
  n : nat
  H : All2 Q [] l''
  H0 := {| pr1 := {| pr1 := []; pr2 := l'' |}; pr2 := H |}
     : sigma
         (λ index : sigma (λ _ : list B, list C),
            All2 Q (pr1 index) (pr2 index))
  H1 := {| pr1 := []; pr2 := l'' |} : sigma (λ _ : list B, list C)
  H2 := H
     : (λ index : sigma (λ _ : list B, list C),
          All2 Q (pr1 index) (pr2 index)) H1
  ============================
   (let H := CoreTactics.block in
    let H0 := CoreTactics.block in
    (∀ (n : nat) (x : A) (y : B) (z : C), P n x y → Q y z → R y z)
    → All2 R [] l'')


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.