def __init__(self, node, filter_=None, stop=None, maxlevel=None):
self.node = node<EOL>self.filter_ = filter_<EOL>self.stop = stop<EOL>self.maxlevel = maxlevel<EOL>self.__iter = None<EOL>
Base class for all iterators.

Iterate over tree starting at `node`.

Keyword Args:
    filter_: function called with every `node` as argument, `node` is returned if `True`.
    stop: stop iteration at `node` if `stop` function returns `True` for `node`.
    maxlevel (int): maximum decending in the node hierarchy.