confirmation before overwriting an existing file.  Interactively,

confirmation is required unless you supply a prefix argument."

  (interactive (list (read-file-name "Save session as: ")

                     (not current-prefix-arg)))

  (when (and confirm (file-exists-p filename))

    (unless (y-or-n-p (format "File `%s' exists; overwrite? " filename))

      (error "Cancelled")))

  (unless (file-writable-p filename)

    (error "File %s can't be written to" filename))

  (wg-perform-session-maintenance)

  (setf (wg-session-file-name (wg-current-session)) filename)

  (setf (wg-session-version (wg-current-session)) wg-version)

  ;; Save opened frames as a session parameter "frame-list".

  ;; Exclude `selected-frame' and daemon one (if any).

  ;; http://stackoverflow.com/questions/21151992/why-emacs-as-daemon-gives-1-more-frame-than-is-opened

  (if wg-control-frames

      (let ((fl (frame-list)))

        ;; TODO: remove using dash

        (mapc (lambda (frame)

                (if (string-equal "initial_terminal" (terminal-name frame))

                    (delete frame fl))) fl)

        (setq fl (delete (selected-frame) fl))

        (let (wg-flag-modified)

  (wg-write-sexp-to-file (wg-pickel-all-session-parameters) filename)

  (wg-fontified-message (:cmd "Wrote: ") (:file filename))

  (wg-mark-everything-unmodified))

(defalias 'wg-write-session-file 'wg-save-session-as)

(defun wg-get-session-file ()

  "Return the filename in which to save the session."

  (or (aif (wg-current-session t) (wg-session-file-name it))

      wg-session-file))

;;(read-file-name (format "Save session as [%s]: " wg-session-file))

(defun wg-save-session (&optional force)

  "Save the current Workgroups session if it's been modified.

When FORCE - save session regardless of whether it's been modified."

  (interactive "P")

  (if (and (not (wg-modified-p)) (not force))

      (wg-message "(The session is unmodified)")

    (wg-save-session-as (wg-get-session-file))))

(defun wg-reset-internal (&optional session)

  "Reset Workgroups, setting `wg-current-session' to SESSION.

Resets all frame parameters, buffer-local vars, current session

object, etc.  SESSION nil defaults to a new, blank session."

  (mapc 'wg-reset-frame (frame-list))

            :test 'string=))

(defun wg-modified-p ()

  "Return t when the current session or any of its workgroups are modified."

  (aif (wg-current-session t)

      (or (wg-session-modified it)

          (cl-some 'wg-workgroup-modified (wg-workgroup-list)))))

(defun wg-mark-everything-unmodified ()

  "Mark the session and all workgroups as unmodified."

  (setf (wg-session-modified (wg-current-session)) nil)

  (dolist (workgroup (wg-workgroup-list))

    (setf (wg-workgroup-modified workgroup) nil)))

(defun wg-session-parameter (parameter &optional default session)

  "Return session's value for PARAMETER.

If PARAMETER is not found, return DEFAULT which defaults to nil.

SESSION nil defaults to the current session."

  (wg-aget (wg-session-parameters (or session (wg-current-session)))

           parameter default))

(defun wg-set-session-parameter (parameter value &optional session)

  "Set PARAMETER to VALUE in SESSION.

SESSION nil means use the current session.  Return value."

    (wg-set-parameter (wg-session-parameters session) parameter value)

    (wg-flag-session-modified session)

    value))

(defun wg-remove-session-parameter (parameter &optional session)

  "Remove parameter PARAMETER from SESSION's parameters."

  (let ((session (or session (wg-current-session))))

    (wg-asetf (wg-session-parameters session) (wg-aremove it parameter))

    (wg-flag-session-modified session)))

(defun wg-session-local-value (variable &optional session)

  "Return the value of VARIABLE in SESSION.

SESSION nil defaults to the current session.  If VARIABLE does

not have a session-local binding in SESSION, the value is

resolved by Emacs."

  (let* ((undefined (cl-gensym))

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

    (if (not (eq value undefined)) value

      (symbol-value variable))))

(defun wg-reset-frame (frame)

  "Reset Workgroups' `frame-parameters' in FRAME to nil."

  (set-frame-parameter frame 'wg-workgroup-state-table nil)

  (set-frame-parameter frame 'wg-current-workgroup-uid nil)