Goal:
  
  A : Type
  P : A → Type
  Q : A → Type
  x : A
  l : list A
  q : Q x
  Hl' : All Q l
  ============================
   ([] = x :: l
    → let H := CoreTactics.block in
      let H0 := CoreTactics.block in All (λ x : A, P x × Q x) [])


DEBUG:Going to execute:
simplify * || simplify ?