The contents of this distribution are "essentially in the public domain".
The scare quotes there are intentional.  For a more precise description,
continue reading.

John Colagioia's original license text can be found in `doc/license.txt`.
(In it, the phrase "this file" referred to the archive in which all the Thue
files were distributed.)  He placed the files he wrote (implementation, spec,
and examples) into the public domain *except* with the proviso that he be
contacted by anyone who wanted to do "anything interesting" with the files,
including modifying them.

Unfortunately, the email address he left for this purpose, with the claim
that it would be kept current, is no longer active.  Thus, one can probably
reasonably assume that he is no longer interested in what happens to his
Thue sources, and that they are unconditionally in the public domain;
however, I am not a lawyer, etc. etc.

Frédéric van der Plancke's implementation, and example source, are much less
questionably in the public domain; his only proviso is that he be given
credit for what he wrote (and not be given credit for any changes or
additions that he did not write.)  Actually, as I understand it (but again,
I am not a lawyer etc etc), putting something into the public domain does
not take away one's moral rights to be recognized as author and to not be
miscredited, so this isn't very much (if at all) different from just being
in the public domain.

I, Chris Pressey, hereby place my implementation in Ruby, my example Thue
programs in `eg`, and the contents of the README, into the public domain,
under the Creative Commons CC0 1.0 Public Domain Dedication, the details
of which can be found here:

    http://creativecommons.org/publicdomain/zero/1.0/

The following example files were taken from the esolangs.org wiki, and
are thus also under the CC0 1.0 Public Domain Dedication:

    truth-machine.t