add_message '[id]="duplicate"' '[subject]="message 0" [filename]=copy0'
test_begin_subtest 'at most 1 thread-id per xapian document'
-test_subtest_known_broken
db=${MAIL_DIR}/.notmuch/xapian
for doc in $(xapian-delve -1 -t '' "$db" | grep '^[1-9]'); do
xapian-delve -1 -r "$doc" "$db" | grep -c '^G'