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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 applyns(self, ns):
 """ Apply the namespace to this node. If the prefix is I{None} then this element's explicit namespace I{expns} is set to the URI defined by I{ns}. Otherwise, the I{ns} is simply mapped. @param ns: A namespace. @type ns: (I{prefix}, I{URI}) """

        if ns is None:
            return
        if not isinstance(ns, (list, tuple)):
            raise Exception("namespace must be a list or a tuple")
        if ns[0] is None:
            self.expns = ns[1]
        else:
            self.prefix = ns[0]
            self.nsprefixes[ns[0]] = ns[1]