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


DEBUG:Going to execute:
clear id' id