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'2 := pr2 Hl'0 : (λ index : list A, All Q index) (pr1 Hl'0)
  ============================
   (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:
set (id'1 := pr1 id') in id'2
DEBUG LTAC Evaluated term: (pr2 Hl'0)