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 : (λ index : list A, All Q index) Hl'1
  ============================
   (Hl' = Hl'2
    → 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:
clear id' id