def footer(preamble_toc, cfr_toc, full_id):
    """Generate "navigation" context which allows the user to move between
    sections in the footer"""
    items = preamble_toc + cfr_toc
    nav = {'previous': None, 'next': None, 'page_type': 'preamble-section'}
    for idx, item in enumerate(items):
        if item.markup_id == full_id:
            if idx > 0:
                nav['previous'] = items[idx - 1]
            if idx < len(items) - 1:
                nav['next'] = items[idx + 1]
    return nav
