/usr/share/acl2-6.3/books/system/ is in acl2-books-source 6.3-5.
This file is owned by root:root, with mode 0o755.
..
/usr/share/acl2-6.3/books/system/compare-out-files.lisp
/usr/share/acl2-6.3/books/system/convert-normalized-term-to-pairs.lisp
/usr/share/acl2-6.3/books/system/deps-pcert.lisp
/usr/share/acl2-6.3/books/system/extend-pathname.lisp
/usr/share/acl2-6.3/books/system/f-put-global.lisp
/usr/share/acl2-6.3/books/system/gather-dcls.lisp
/usr/share/acl2-6.3/books/system/hl-addr-combine.lisp
/usr/share/acl2-6.3/books/system/legal-variablep.lisp
/usr/share/acl2-6.3/books/system/meta-extract.lisp
/usr/share/acl2-6.3/books/system/origin.lisp
/usr/share/acl2-6.3/books/system/pcert/
/usr/share/acl2-6.3/books/system/pcert/acl2x-pcert-test-1.acl2
/usr/share/acl2-6.3/books/system/pcert/acl2x-pcert-test-1.lisp
/usr/share/acl2-6.3/books/system/pcert/acl2x-pcert-test-2-include.lisp
/usr/share/acl2-6.3/books/system/pcert/acl2x-pcert-test-2.acl2
/usr/share/acl2-6.3/books/system/pcert/acl2x-pcert-test-2.lisp
/usr/share/acl2-6.3/books/system/pcert/cert.acl2
/usr/share/acl2-6.3/books/system/pcert/elide-test.lisp
/usr/share/acl2-6.3/books/system/pcert/mid.acl2
/usr/share/acl2-6.3/books/system/pcert/mid.lisp
/usr/share/acl2-6.3/books/system/pcert/pkg-test.acl2
/usr/share/acl2-6.3/books/system/pcert/pkg-test.lisp
/usr/share/acl2-6.3/books/system/pcert/sub.lisp
/usr/share/acl2-6.3/books/system/pcert/top.acl2
/usr/share/acl2-6.3/books/system/pcert/top.lisp
/usr/share/acl2-6.3/books/system/pseudo-good-worldp.lisp
/usr/share/acl2-6.3/books/system/pseudo-termp-lemmas.lisp
/usr/share/acl2-6.3/books/system/random.lisp
/usr/share/acl2-6.3/books/system/subcor-var.lisp
/usr/share/acl2-6.3/books/system/sublis-var.lisp
/usr/share/acl2-6.3/books/system/subst-expr.lisp
/usr/share/acl2-6.3/books/system/subst-var.lisp
/usr/share/acl2-6.3/books/system/too-many-ifs.acl2
/usr/share/acl2-6.3/books/system/too-many-ifs.lisp
/usr/share/acl2-6.3/books/system/top.lisp
/usr/share/acl2-6.3/books/system/update-state.lisp
/usr/share/acl2-6.3/books/system/verified-termination-and-guards.lisp
/usr/share/acl2-6.3/books/system/worldp-check.acl2
/usr/share/acl2-6.3/books/system/worldp-check.lisp