code: def get_or_create_element(self, ns, name):
          
          if len(self._node.xpath('%s:%s' % (ns, name), namespaces=SLDNode._nsmap)) == 1:
              return getattr(self, name)
  
          return self.create_element(ns, name)
docs: Attempt to get the only child element from this SLDNode. If the node
          does not exist, create the element, attach it to the DOM, and return
          the class object that wraps the node.
  
          @type    ns: string
          @param   ns: The namespace of the new element.
          @type  name: string
          @param name: The name of the new element.
          @rtype: L{SLDNode}
          @return: The wrapped node, in the parent's property class. This will
                   always be a descendent of SLDNode.