def is_noun(pos_tag):
    """Return ``True`` if ``pos_tag`` is ``NOUN`` or ``PROPN``, False otherwise.

    :param pos_tag: Part of Speech tag
    :type pos_tag: string
    :return: True if POS is noun or proper noun
    :rtype: boolean
    """
    return pos_tag == 'PROPN' or pos_tag == 'NOUN'
