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 := Hl' : (λ index : list A, All Q index) Hl'1
  ============================
   (let H := CoreTactics.block in
    let H0 := CoreTactics.block in All (λ x : A, P x × Q x) [])


DEBUG:Going to execute:
generalize (eq_refl : t = id'1); clearbody id'2 id'1; clear id' id;
            try unfold signature in id'2