code: def _add_dominance_relation(self, source, target):
          
          # TODO: fix #39, so we don't need to add nodes by hand
          self.add_node(target, layers={self.ns, self.ns+':unit'})
          self.add_edge(source, target,
                        layers={self.ns, self.ns+':discourse'},
                        edge_type=EdgeTypes.dominance_relation)
docs: add a dominance relation to this docgraph