# The folder's file list is the top level of ONE table of contents:
# level 0 rows are files, deeper rows are those files' own sections.
level = corpus_toc[corpus_toc.level == 0]           # 63 rows, one per file
files = reason_on_toc(question, level,              # the SAME router call
                      section_signals=keyword_hits(question, level))

sections = []
for file_id in files.section_ids:                   # typically 1 to 3
    level = corpus_toc[(corpus_toc.file_id == file_id)
                       & (corpus_toc.level == 1)]
    while True:                                     # the one-document loop
        pick    = reason_on_toc(question, level)
        section = level[level.section_id.isin(pick.section_ids)]
        kids    = immediate_children(corpus_toc, section)
        if kids.empty or section.n_pages.max() <= SMALL:
            break                                   # a leaf, or short enough
        level = kids                                # open it, descend
    sections.append(section)

# 63 files -> 1 file -> AC-2 ACCOUNT MANAGEMENT -> 5 pages out of 4,211