@classmethod
def _get_targets(cls, tree):
    """Get targets of a tree

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

        Yields
        ------
            int
                the identifier of target nodes.
            -2
                if the tree is not VP and pos of the first leaf node starts with 'Cb'
            -1
                if the tree is not VP

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

            1. POS-tag is `Nh`
            2. is one of the pronoun words from E-HowNet

        """
    root = tree[tree.root]
    leaves = tree.leaves()
    if root.data.pos == 'VP':
        if leaves[0].data.pos.startswith('Cb'):
            yield -2
        else:
            yield -1
    for node in leaves:
        if cls._is_pronoun_word(node):
            yield node.identifier
