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(define-key ac-menu-map (kbd "SPC")
  (defun ac-complete-with-space ()
    "Select the candidate and append a space. save your time for typing space."
    (interactive)
    (ac-complete)
    (insert " ")
    ;; ;; FIXME: auto insert two spaces.
    ;; (if (pre-char before point is *only* not whitespace)
    ;;     (insert " "))
    ))
(add-hook 'org-mode-hook
          (lambda ()
            (define-key ac-menu-map (kbd "SPC") 'self-insert-command)))