Goal:
  
  A : Type
  P : A → Type
  Q : A → Type
  x : A
  l : list A
  p : P x
  Hl : All P l
  Hl' : All Q (x :: l)
  Hl'0 := {| pr1 := x :: l; pr2 := Hl' |}
       : sigma (λ index : list A, All Q index)
  Hl'1 := x :: l : list A
  Hl'2 := Hl' : (λ index : list A, All Q index) Hl'1
  ============================
   (let H := CoreTactics.block in
    (All Q l → All (λ x : A, P x × Q x) l)
    → let H0 := CoreTactics.block in All (λ x : A, P x × Q x) (x :: 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.