docs: 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