/usr/share/acl2-7.1/books/tools/flag.acl2 is in acl2-books-source 7.1-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 | (include-book "std/portcullis" :dir :system)
(certify-book "flag" ? t)
|