@classmethod
def _get_sources(cls, tree):
    """Get sources of a tree

        Parameters
        ----------
            tree : :class:`ParsedTree <ckipnlp.container.util.parsed_tree.ParsedTree>`
                the parser tree.

        Yields
        ------
            int
                the identifier of source nodes.

        Notes
        -----
            A node can be a coreference source if either:

            1. POS-tag is `Nb`
            2. is one of the human words from E-HowNet

        """
    for node in tree.leaves():
        if cls._is_human_word(node):
            yield node.identifier
