Return a new PileupCollection instance including only pileups for 
        the specified loci.

def at(self, *loci):
        '''
        Return a new PileupCollection instance including only pileups for 
        the specified loci.
        '''
        loci = [to_locus(obj) for obj in loci]
        single_position_loci = []
        for locus in loci:
            for position in locus.positions:
                single_position_loci.append(
                    Locus.from_interbase_coordinates(locus.contig, position))
        pileups = dict(
            (locus, self.pileups[locus]) for locus in single_position_loci)
        return PileupCollection(pileups, self)