Goal:
  
  A : Type
  B : Type
  P : A → B → Type
  Q : A → B → Type
  x : A
  y : B
  l : list A
  l' : list B
  r : P x y
  X : All2 P l l'
  IHX : All2 Q l l' → All2 (Trel_conj P Q) l l'
  H : All2 Q (x :: l) (y :: l')
  H0 := {| pr1 := {| pr1 := x :: l; pr2 := y :: l' |}; pr2 := H |}
     : sigma
         (λ index : sigma (λ _ : list A, list B),
            All2 Q (pr1 index) (pr2 index))
  H1 := {| pr1 := x :: l; pr2 := y :: l' |} : sigma (λ _ : list A, list B)
  H2 := H
     : (λ index : sigma (λ _ : list A, list B),
          All2 Q (pr1 index) (pr2 index)) H1
  ============================
   (let H := CoreTactics.block in
    let H0 := CoreTactics.block in All2 (Trel_conj P Q) (x :: l) (y :: 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.