code: def _identify(self, dataframe):
          
          ##
          # TODO Fix this implementation. (i.e., why not support just 'left')
          # At the moment this implementation won't work at all.
          # The logic here can be simplified.
          id1 = dataframe[self.channels[0]] >= self.vert[0]
          id2 = dataframe[self.channels[1]] >= self.vert[1]
  
          if 'left' in self.region: id1 = ~id1
          if 'bottom' in self.region: id2 = ~id2
  
          idx = id1 & id2
  
          if 'out' in self.region:
              idx = ~idx
  
          return idx
docs: Returns a list of indexes containing only the points that pass the filter.
  
          Parameters
          ----------
          dataframe : DataFrame