/usr/share/acl2-6.3/books/ordinals/ is in acl2-books-certs 6.3-5.
This file is owned by root:root, with mode 0o755.
..
/usr/share/acl2-6.3/books/ordinals/e0-ordinal.cert
/usr/share/acl2-6.3/books/ordinals/lexicographic-book.cert
/usr/share/acl2-6.3/books/ordinals/lexicographic-ordering-without-arithmetic.cert
/usr/share/acl2-6.3/books/ordinals/lexicographic-ordering.cert
/usr/share/acl2-6.3/books/ordinals/limits.cert
/usr/share/acl2-6.3/books/ordinals/ordinal-addition.cert
/usr/share/acl2-6.3/books/ordinals/ordinal-basic-thms.cert
/usr/share/acl2-6.3/books/ordinals/ordinal-counter-examples.cert
/usr/share/acl2-6.3/books/ordinals/ordinal-definitions.cert
/usr/share/acl2-6.3/books/ordinals/ordinal-exponentiation.cert
/usr/share/acl2-6.3/books/ordinals/ordinal-isomorphism.cert
/usr/share/acl2-6.3/books/ordinals/ordinal-multiplication.cert
/usr/share/acl2-6.3/books/ordinals/ordinal-total-order.cert
/usr/share/acl2-6.3/books/ordinals/ordinals-without-arithmetic.cert
/usr/share/acl2-6.3/books/ordinals/ordinals.cert
/usr/share/acl2-6.3/books/ordinals/proof-of-well-foundedness.cert
/usr/share/acl2-6.3/books/ordinals/top-with-meta.cert