Major Section: BOOKS
For relevant background see books, see certificate, and see portcullis.
Include-book has a special provision for dealing with an uncertified
book, i.e., a file with no certificate or an invalid
certificate (i.e., one whose check sums describe files other than the
ones actually read). In this case, a warning is printed and the book is
otherwise processed much as though it were certified and had an open
portcullis.
If a book B.lisp is uncertified and a file B.port exists, then the
forms in B.port are evaluated before the forms in B.lisp. Such a
file B.port is typically created calling certify-book on book
"B" with argument :write-port t, so that B.port contains the
portcullis commands for B (the commands present in the
world when that certification was attempted).
Inclusion of uncertified books can be handy, but it can have disastrous consequences.
The provision allowing uncertified books to be included can have disastrous consequences, ranging from hard lisp errors, to damaged memory, to quiet logical inconsistency.
It is possible for the inclusion of an uncertified book to render the logic
inconsistent. For example, one of its non-local events might be
(defthm t-is-nil (equal t nil)). It is also possible for the inclusion
of an uncertified book to cause hard errors or breaks into raw Common
Lisp. For example, if the file has been edited since it was certified, it
may contain too many open parentheses, causing Lisp to read past ``end of
file.'' Similarly, it might contain non-ACL2 objects such as 3.1415 or
ill-formed event forms that cause ACL2 code to break.
Even if a book is perfectly well formed and could be certified (in a suitable
extension of ACL2's initial world), its uncertified inclusion might
cause Lisp errors or inconsistencies! For example, it might mention packages
that do not exist in the host world, especially if the .port file
(discussed above) does not exist from an earlier certification attempt. The
portcullis of a certified book ensures that the correct defpkgs
have been admitted, but if a book is read without actually raising its
portcullis, symbols in the file, e.g., acl2-arithmetic::fn, could
cause ``unknown package'' errors in Common Lisp. Perhaps the most subtle
disaster occurs if the host world does have a defpkg for each
package used in the book but the host defpkg imports different symbols
than those required by the portcullis. In this case, it is possible
that formulas which were theorems in the certified book are non-theorems in
the host world, but those formulas can be read without error and will
then be quietly assumed.
In short, if you include an uncertified book, all bets are off regarding the validity of the future behavior of ACL2.
That said, it should be noted that ACL2 is pretty tough and if errors don't occur, the chances are that deductions after the inclusion of an uncertified book are probably justified in the (possibly inconsistent) logical extension obtained by assuming the admissibility and validity of the definitions and conjectures in the book.
