code: def bib_keys(self):
          
          bib_keys = []
          # Get bib keys in this document
          for match in texutils.cite_pattern.finditer(self.text):
              keys = match.group(5).split(',')
              bib_keys += keys
  
          # Recursion
          for path, document in self._children.iteritems():
              bib_keys += document.bib_keys
          bib_keys = list(set(bib_keys))
  
          return bib_keys
docs: List of all bib keys in the document (and input documents).