Goal:
  
  A : Type
  B : Type
  P : A → Type
  Q : A → B → Type
  H2 : ∀ (x : A) (y : B), In y [] → P x → Q x y
  Hl : 0 = 0
  Hl0 := {| pr1 := 0; pr2 := Hl |} : sigma (λ index : nat, 0 = index)
  Hl1 := 0 : nat
  Hl2 := Hl : (λ index : nat, 0 = index) Hl1
  ============================
   (let H := CoreTactics.block in let H0 := CoreTactics.block in All2 Q [] [])


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.