(let* ((undefined (wg-gensym))

             (value (wg-workgroup-parameter workgroup variable undefined)))

        (if (not (eq value undefined)) value

          (wg-session-local-value variable))))))

(defalias 'wg-local-value 'wg-workgroup-local-value)

;;; workgroup associated buffers

(defun wg-workgroup-associated-buf-uids (workgroup)

  "Return a new list containing all of WORKGROUP's associated buf uids."

  (append (wg-workgroup-strong-buf-uids workgroup)

          (wg-workgroup-weak-buf-uids workgroup)))

(defun wg-workgroup-associated-bufs (workgroup)

  "Return a new list containing all of WORKGROUP's associated bufs."

  (delete nil (mapcar 'wg-find-buf-by-uid

                      (wg-workgroup-associated-buf-uids workgroup))))

(defun wg-workgroup-associated-buffers (workgroup &optional initial names)

  "Return a list of WORKGROUP's live associated buffers."

  (let ((assoc-bufs (wg-workgroup-associated-bufs workgroup)))

     (lambda (buffer) (wg-find-buffer-in-buf-list buffer assoc-bufs))

     (or initial (buffer-list)))))

(defun wg-workgroup-bufobj-association-type (workgroup bufobj)

  "Return BUFOBJ's association-type in WORKGROUP, or nil if unassociated."

  (let ((uid (wg-bufobj-uid-or-add bufobj)))

    (or (and (member uid (wg-workgroup-strong-buf-uids workgroup)) 'strong)

        (and (member uid (wg-workgroup-weak-buf-uids workgroup)) 'weak))))

(defun wg-workgroup-strongly-associate-bufobj (workgroup bufobj)

  "Strongly associate BUFOBJ with WORKGROUP."

  (let* ((uid (wg-bufobj-uid-or-add bufobj))

         (remp (wg-removef-p uid (wg-workgroup-weak-buf-uids workgroup)

                             :test 'string=))

         (addp (wg-pushnew-p uid (wg-workgroup-strong-buf-uids workgroup)

                             :test 'string=)))

    (when (or remp addp)

      (wg-flag-workgroup-modified workgroup)

      bufobj)))

(defun wg-workgroup-weakly-associate-bufobj (workgroup bufobj)

  "Weakly associate BUFOBJ with WORKGROUP."

  (let* ((uid (wg-bufobj-uid-or-add bufobj))

         (remp (wg-removef-p uid (wg-workgroup-strong-buf-uids workgroup)

It defaults to `wg-get-buffer-list-filter-val'.

INITIAL non-nil should be an initial buffer-list to filter.  It defaults to

`wg-interesting-buffers'."

  (let ((buffer-list (funcall (wg-get-buffer-list-filter-val

                               (wg-get-buffer-list-filter-id-flexibly bfl-id)

                               'constructor)

                              (wg-get-workgroup workgroup)

                              (or initial (wg-interesting-buffers)))))

    (if names (mapcar 'wg-buffer-name buffer-list) buffer-list)))

;; buffer-list filters

(defun wg-buffer-list-filter-all (workgroup initial)

  "Return all buffers in INITIAL."

  initial)

(defun wg-buffer-list-filter-associated (workgroup initial)

  "Return only those buffers associated with WORKGROUP."

  (wg-workgroup-associated-buffers workgroup initial))

(defun wg-buffer-list-filter-unassociated (workgroup initial)

  "Return only those buffer unassociated with WORKGROUP."

  (let ((buffers (wg-workgroup-associated-buffers workgroup initial)))

;; buffer-list filtration utils

(defun wg-filter-buffer-list-by-regexp (regexp buffer-list)

  "Return only those buffers in BUFFER-LIST with names matching REGEXP."

                 buffer-list :key 'buffer-name))

(defun wg-filter-buffer-list-by-root-dir (root-dir buffer-list)

  "Return only those buffers in BUFFER-LIST visiting files undo ROOT-DIR."

                 buffer-list :key 'buffer-file-name))

(defun wg-filter-buffer-list-by-major-mode (major-mode buffer-list)

  "Return only those buffers in BUFFER-LIST in major-mode MAJOR-MODE."

                 buffer-list :key 'wg-buffer-major-mode))

;; Example custom buffer-list-filters

(defun wg-buffer-list-filter-irc (workgroup buffer-list)

  "Return only those buffers in BUFFER-LIST with names starting in \"#\"."

  (wg-filter-buffer-list-by-regexp "^#" buffer-list))

(defun wg-buffer-list-filter-home-dir (workgroup buffer-list)

  "Return only those buffers in BUFFER-LIST visiting files under ~/."

  (wg-filter-buffer-list-by-root-dir "~/" buffer-list))

(defun wg-buffer-list-filter-elisp (workgroup buffer-list)

  "Return only those buffers in BUFFER-LIST in `emacs-lisp-mode'."

  (wg-filter-buffer-list-by-major-mode 'emacs-lisp-mode buffer-list))

(defun wg-buffer-list-filter-home-dir->elisp (workgroup buffer-list)

  "Example of chaining buffer-list filters.

This returns all buffers under \"~/\" that are also in `emacs-lisp-mode'."

  (wg-buffer-list-filter-elisp

   nil (wg-buffer-list-filter-home-dir nil buffer-list)))

;;; garbage collection

;; update buf list

(defun wg-update-buffer-in-buf-list (&optional buffer)

  "Update BUFFER's corresponding buf in `wg-buf-list'.

BUFFER nil defaults to `current-buffer'."

  (let ((buffer (or buffer (current-buffer))))

    (wg-when-let ((uid (wg-buffer-uid buffer))

                  (old-buf (wg-find-buf-by-uid uid))

                  (new-buf (wg-buffer-to-buf buffer)))

      (setf (wg-buf-uid new-buf) (wg-buf-uid old-buf))

      (wg-asetf (wg-buf-list) (cons new-buf (remove old-buf it))))))

(defun wg-update-buf-list (&optional buffer-list)

  "Update all bufs in `wg-buf-list' corresponding to buffers in BUFFER-LIST."

  (mapc 'wg-update-buffer-in-buf-list (or buffer-list (buffer-list))))

;; gc buf uids

(defun wg-workgroup-gc-buf-uids (workgroup)

  "Remove buf uids from WORKGROUP that have no referent in `wg-buf-list'."

  (wg-asetf (wg-workgroup-strong-buf-uids workgroup)

            (wg-workgroup-weak-buf-uids workgroup)

(defun wg-gc-buf-uids ()

  "Remove from all workgroups those buf uids that have no referent in `wg-buf-list'."

  (mapc 'wg-workgroup-gc-buf-uids (wg-workgroup-list)))

;; gc bufs

(defun wg-wtree-buf-uids (wtree)

  "Return a new list of the buf uids of all wins in wtree."

  (wg-flatten-wtree wtree 'wg-win-buf-uid))

(defun wg-wtree-unique-buf-uids (wtree)

  "Return a list of the unique buf uids of all wins in wtree."

  (remove-duplicates (wg-wtree-buf-uids wtree) :test 'string=))

(defun wg-wconfig-buf-uids (wconfig)

  "Return WCONFIG's wtree's `wg-wtree-buf-uids'."

  (wg-wtree-unique-buf-uids (wg-wconfig-wtree wconfig)))

(defun wg-workgroup-base-wconfig-buf-uids (workgroup)

  "Return a new list of all unique buf uids in WORKGROUP's working wconfig."

  (wg-wconfig-buf-uids (wg-workgroup-base-wconfig workgroup)))

(defun wg-session-all-buf-uids (&optional session)

  "Return a new list of all unique buf uids in SESSION.

SESSION nil defaults to `wg-current-session'."

  (reduce 'wg-string-list-union

          (wg-session-workgroup-list (or session (wg-current-session)))

          :key 'wg-workgroup-all-buf-uids))

(defun wg-buffer-list-all-uids (&optional buffer-list)

  "Return a list of the uids of all buffers in BUFFER-LIST in

which `wg-buffer-uid' is locally bound.

BUFFER-LIST nil defaults to `buffer-list'."

  (delq nil (mapcar 'wg-buffer-uid (or buffer-list (buffer-list)))))

(defun wg-all-buf-uids (&optional session buffer-list)

  "Return the union of `wg-session-all-buf-uids' and `wg-buffer-list-all-uids'."

  (union (wg-session-all-buf-uids session)

         (wg-buffer-list-all-uids buffer-list)

         :test 'string=))

(defun wg-gc-bufs ()

  "gc bufs from `wg-buf-list' that are no longer needed."

  (let ((all-buf-uids (wg-all-buf-uids)))

    (wg-asetf (wg-buf-list)

                             :key 'wg-buf-uid))))

;; FIXME: Duplicate buf names probably shouldn't be allowed.  An unrelated error

;; causes two *scratch* buffers to be present, triggering the "uids don't match"

;; error.  Write something to remove bufs with duplicate names.

(defun wg-perform-session-maintenance ()

  "Perform various maintenance operations on the current Workgroups session."

  (wg-update-current-workgroup-working-wconfig)

  (wg-update-all-base-wconfigs)

  (wg-gc-bufs)

  (wg-gc-buf-uids)

  (wg-update-buf-list))

;; session consistency testing

(defun wg-session-uids-consistent-p ()

  "Return t if there are no duplicate bufs or buf uids in the wrong places,

nil otherwise."

  (and (every (lambda (wg)

                (not (wg-dups-p (wg-workgroup-associated-buf-uids wg)