Get a node from a node coord. 

        :param coord: the coordinates of the required node.
        :type coord: tuple or list
        :param relative: `True` if coord is relative to the node instance,
            `False` for absolute coordinates.
        :type relative: bool
        :returns: the node corresponding to `coord`.
        :rtype: Node or None

def get_node_by_coord(self, coord, relative=False):
        """Get a node from a node coord. 

        :param coord: the coordinates of the required node.
        :type coord: tuple or list
        :param relative: `True` if coord is relative to the node instance,
            `False` for absolute coordinates.
        :type relative: bool
        :returns: the node corresponding to `coord`.
        :rtype: Node or None
        """
        if not isinstance(coord, (list, tuple)) or False in list(map(lambda i: type(i)==int, coord)):
            logger.warning("%s.get_node_by_coord: node«%s», arg «coord»=«%s», «coord» must be list or tuple of integers." % (self.__class__.__name__, self.name, coord))
            return None
        if relative:
            _node = self
        else:
            _node = self._root # _node = self.get_rootnode()
        for idx in coord:
            _node = _node.childs[idx]
            if _node is None:
                logger.warning("%s.get_node_by_coord: node«%s», arg «coord»=«%s» not valid." % (self.__class__.__name__, self.name, coord))
                return None
        return _node