def _identify(self, dataframe):
        """
        Returns a list of indexes containing only the points that pass the filter.

        Parameters
        ----------
        dataframe : 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
Returns a list of indexes containing only the points that pass the filter.

        Parameters
        ----------
        dataframe : DataFrame