anchor.diff:  
   (defun make-bit-vector (size &key (allocation :heap))
     (assert (member allocation '(nil :heap :static)) nil
  -    (error "The `allocation` should be either :heap or :static, or it's set to :heap by default."))
  +          (error "The `allocation` should be either :heap or :static, or it's set to :heap by default."))
     (let ((params `(,size :element-type bit :initial-element 0)))
       (apply (case allocation
                ((nil :heap) 'make-array)
                (:static 'static-vectors:make-static-vector))
  -           params)))
  +                      params)))
   
   (defclass bloom-filter ()
     ((array :accessor filter-array :initarg :array :type simple-bit-vector)
  -   (array-static-p :accessor filter-array-static-p :initarg :array-static-p :type symbol)
  +   (%array-static-p% :accessor filter-array-static-p :initarg :array-static-p :type symbol)
      (order :accessor filter-order :initarg :order :type integer)
      (degree :accessor filter-degree :initarg :degree :type integer)
      (seed :accessor filter-seed :initarg :seed :type integer
  
positive.code: 
  (defun destroy-filter (filter)
    "Destroy a Bloom filter instance. When its bit array is allocated statically,
  then free the memory and set the reference of each slot to nil;
  otherwise, set all the references of slots to nil."
    (with-slots (array array-static-p order degree seed) filter
      (setf order nil degree nil seed nil)
      (if array-static-p
          (static-vectors:free-static-vector array)
          (setf array nil))
      (setf array nil array-static-p nil))
    filter)
  
  (defun filter-union (filter1 filter2)
    "Return the union of FILTER1 and FILTER2 as a new filter."
  
negative.code: (defmethod initialize-instance :after ((filter bloom-filter) &key order static)
    (setf (slot-value filter 'array)
          (make-bit-vector order :allocation (if static :static :heap))
          (slot-value filter '%array-static-p%)
          static))
  
  (defun bloom-filter-p (object)
    (typep object 'bloom-filter))
  