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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 create(self, tag, value):
 u"""Creates a node, if it doesn't exist yet. Unlike attribute access, this allows to pass a node's name with hyphens. Those hyphens will be normalized automatically. In case the required element already exists, raises an exception. Updating/overwriting should be done using `update``. """

        child_tags = {child.tag for child in self._xml}

        if tag in child_tags:
            raise KeyError('Node {} already exists in XML tree.'.format(tag))

        self.set(tag, value)