When comparing free and commercial Prolog implementations, a rule of
thumb is that commercial prolog implementations are often more robust
and better supported than the public domain and free prolog
implementations. Commercial Prolog implementations tend to have better
debugging facilities. Many of the commercial Prolog vendors offer
educational discounts to universities, and some of the commercial
Prolog implementations are rather inexpensive. 

When considering a commercial Prolog implementation, be sure to ask
for current pricing information. Although we try to keep this
information up to date, there is no guarantee that it hasn't changed
in the interim. If you find that the information has changed, please
ask the vendor to send us current information.

Some research institutions make their Prolog implementations available
for a fee. We have included those implementations in the lists of
commercial Prolog implementations.

The Prolog Management Group may be contacted by email via the Secretary,
at <info@pap.com>.

Remember when ftping compressed or compacted files (.Z, .arc, .fit,
.zip, .z, etc.) to use binary mode for retrieving the files.

Files that end with a .z suffix were compressed with the patent-free
gzip (no relation to zip). Source for gzip is available from:
   ftp://prep.ai.mit.edu/pub/gnu/
as the files gzip-1.2.3.shar, gzip-1.2.3.tar,or gzip-1.2.3.msdos.exe.