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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 prefix2ns(self, prefix: YangIdentifier, mid: ModuleId) -> YangIdentifier: """Return the namespace corresponding to a prefix. Args: prefix: Prefix associated with a module and its namespace. mid: Identifier of the module in which the prefix is declared. Raises: ModuleNotRegistered: If `mid` is not registered in the data model. UnknownPrefix: If `prefix` is not declared. """

        try:
            mdata = self.modules[mid]
        except KeyError:
            raise ModuleNotRegistered(*mid) from None
        try:
            return mdata.prefix_map[prefix][0]
        except KeyError:
            raise UnknownPrefix(prefix, mid) from None