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)
  ============================
   (let H := CoreTactics.block in
    let H0 := CoreTactics.block in All (λ x : A, P x × Q x) [])


DEBUG:Going to execute:
set (id'2 := pr2 id'); set (id'1 := pr1 id') in id'2