def set_class_on_child(self, node, class_, index=0):
        children = [n for n in node if not isinstance(n, nodes.Invisible)]
        try:
            child = children[index]
        except IndexError:
            return
        child['classes'].append(class_)
Set class `class_` on the visible child no. index of `node`.
dataset/ETHPy150Open zackw/header-survey/sphinx/ext/html5_output.py/BaseTranslator.set_class_on_child