Append a PileupElement to this Pileup. If an identical PileupElement is
        already part of this Pileup, do nothing.

def append(self, element):
        '''
        Append a PileupElement to this Pileup. If an identical PileupElement is
        already part of this Pileup, do nothing.
        '''
        assert element.locus == self.locus, (
            "Element locus (%s) != Pileup locus (%s)"
            % (element.locus, self.locus))
        self.elements[element] = None