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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 istoken(docgraph, node_id, namespace=None):
 """returns true, iff the given node ID belongs to a token node. Parameters node_id : str the node to be checked namespace : str or None If a namespace is given, only look for tokens in the given namespace. Otherwise, look for tokens in the default namespace of the given document graph. """

    if namespace is None:
        namespace = docgraph.ns
    return namespace+':token' in docgraph.node[node_id]