def segment(self):
        """
        If only one segment in the segmentlist this will be that segment.
        Otherwise an error is raised.
        """
        if len(self.segment_list) == 1:
            return self.segment_list[0]
        else:
            err = "self.segment_list must only contain one segment to access"
            err += " the segment property. %s." %(str(self.segment_list),)
            raise TypeError(err)
If only one segment in the segmentlist this will be that segment.
        Otherwise an error is raised.