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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def _add_dominance_relation(self, source, target):
 """add a dominance relation to this docgraph"""

        # 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)