Returns the lineage of histories leading up to `h`.

def lineage(self, h):
        """
        Returns the lineage of histories leading up to `h`.
        """

        lineage = [ ]

        predecessors = list(self._graph.predecessors(h))
        while len(predecessors):
            lineage.append(predecessors[0])
            predecessors = list(self._graph.predecessors(predecessors[0]))

        lineage.reverse()
        return lineage