Given a 1-base locus, return the Pileup at that locus.

        Raises a KeyError if this PileupCollection does not have a Pileup at
        the specified locus.

def pileup(self, locus):
        '''
        Given a 1-base locus, return the Pileup at that locus.

        Raises a KeyError if this PileupCollection does not have a Pileup at
        the specified locus.
        '''
        locus = to_locus(locus)
        if len(locus.positions) != 1:
            raise ValueError("Not a single-base locus: %s" % locus)
        return self.pileups[locus]