(defun delete-empty-lines ()
  "Delete empty lines in bibliography."
  (interactive)
  ;; set point-min
  (setq m1 (make-marker))
  (goto-char (point-min))
  (search-forward "\begin{thebibliography}" nil nil)
  (beginning-of-line)
  (set-marker m1 (point))
  ;; set point-max
  (setq m2 (make-marker))
  (search-forward "\end{thebibliography}" nil nil)
  (end-of-line)
  (set-marker m2 (point))
  ;; set marker
  (push-mark m1 nil t)
  (push-mark m2 nil t)
  ;; color region
  (setq mark-active t)
  ;; clean empty lines
  (flush-lines "$^" m1 m2)
  ;; decolor region
  (setq mark-active nil))