cert_pl_exclude

The presence of this file tells cert.pl not to try to build any of the books in
this directory.

Quoting from workshops/2000/Makefile:

# cowles/books -- tarai4 is resource-demanding: crashes some lisps, overflows
#                 in others.  At some point we may deal with this.
#                 Also, a change will be needed for v2-6 in tarai6, to get
#                 through lemma tarai=Fb-3.

