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


DEBUG:Going to execute:
match goal with
           | |- let _ := CoreTactics.block in ?T =>
                 match T with
                 | context [ CoreTactics.block ] =>
                     change T; CoreTactics.intros_until_block
                 end
           | _ => CoreTactics.unblock_goal
           end