I have written a straight-forward, free to use implementation of the
Iterative Dichotomiser 3 (ID3) algorithm for Common Lisp (see
http://en.wikipedia.org/wiki/ID3_algorithm for a very short description of
the algorithm and its use).
It is ASDF-install'able, BSD-licensed and works on property lists (plists)
for ease of use.
It can be downloaded from:
http://www.pvv.ntnu.no/~oyvinht/static/OSS/cl-id3/
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Oyvin