def walk(self, visitor):
        """
        Walk the branch and call the visitor function
        on each node.
        @param visitor: A function.
        @return: self
        @rtype: L{Element}
        """
        visitor(self)
        for c in self.children:
            c.walk(visitor)
        return self
Walk the branch and call the visitor function
        on each node.
        @param visitor: A function.
        @return: self
        @rtype: L{Element}