def xblock_primary_child_category(xblock):
    """
    Returns the primary child category for the specified xblock, or None if there is not a primary category.
    """
    category = xblock.category
    if category == 'course':
        return 'chapter'
    elif category == 'chapter':
        return 'sequential'
    elif category == 'sequential':
        return 'vertical'
    return None