code: def remove_bookmark(self, name=None, time=None, chan=None):
          
          bookmarks = self.rater.find('bookmarks')
  
          for m in bookmarks:
  
              bookmark_name = m.find('bookmark_name').text
              bookmark_start = float(m.find('bookmark_start').text)
              bookmark_end = float(m.find('bookmark_end').text)
              bookmark_chan = m.find('bookmark_chan').text
              if bookmark_chan is None:  # xml doesn't store empty string
                  bookmark_chan = ''
  
              if name is None:
                  name_cond = True
              else:
                  name_cond = bookmark_name == name
  
              if time is None:
                  time_cond = True
              else:
                  time_cond = (time[0] <= bookmark_end and
                               time[1] >= bookmark_start)
  
              if chan is None:
                  chan_cond = True
              else:
                  chan_cond = bookmark_chan == chan
  
              if name_cond and time_cond and chan_cond:
                  bookmarks.remove(m)
  
          self.save()
docs: if you call it without arguments, it removes ALL the bookmarks.