code: def create(self, tag, value):
          u
          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)
docs: 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``.