title: elisp newbie question: Can't find 'filename' function definition in org.el?
body: <p>I really love org-mode in emacs and want to customize a few things. While reading thru org.el, I'm finding several references to <code>filename</code> but can't find <code>filename</code> using <code>describe-function</code>? </p>
  
  <p>I'm sure there's a simple answer, but I'm just learning elisp and it's not obvious. Any insight into where <code>filename</code> is defined? And/or if it's not a function, what is it?</p>
  
  <p>For example, <code>filename</code> on line 25502: </p>
  
  <pre><code>         (filename (if to-buffer 
           (expand-file-name
            (concat
             (file-name-sans-extension
          (or (and subtree-p
               (org-entry-get (region-beginning)
                      "EXPORT_FILE_NAME" t))
              (file-name-nondirectory buffer-file-name)))
             "." html-extension)
            (file-name-as-directory
             (or pub-dir (org-export-directory :html opt-plist))))))
  </code></pre>