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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 validate(self, graph):
 """ Validate the graph by checking whether it is a directed acyclic graph. Args: graph (DiGraph):
 Reference to a DiGraph object from NetworkX. Raises: DirectedAcyclicGraphInvalid: If the graph is not a valid dag. """

        if not nx.is_directed_acyclic_graph(graph):
            raise DirectedAcyclicGraphInvalid(graph_name=self._name)