docs: def RetrieveObject(self, ui, start=-1, length=-1):
          '''Retrieves the contents of a document.
          This function takes a UnitInfo and two optional arguments, the first
          being the start address and the second is the length. These define
          the amount of data to be read from the archive.
          '''
          if self.file and ui:
              if length == -1:
                  len = ui.length
              else:
                  len = length
              if start == -1:
                  st = 0l
              else:
                  st = long(start)
              return chmlib.chm_retrieve_object(self.file, ui, st, len)
          else:
              return (0, '')