Goal:
  
  A : Type
  B : Type
  C : Type
  Q : C → Type
  P : C → A → Prop
  x : A
  xs : list A
  y : C
  X : Q y
  x0 : A
  l : list A
  p : P y x0
  h : All (P y) l
  ============================
   (x :: xs = x0 :: l
    → let H := CoreTactics.block in
      let H0 := CoreTactics.block in All (P y) xs)


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