docs: 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)