Removed theory NatDef.
* Cambridge (UK)
http://www.cl.cam.ac.uk/Research/HVG/Isabelle/dist/
* Munich (Germany)
http://isabelle.in.tum.de/dist/
* New Jersey (USA)
http://ftp.research.bell-labs.com/dist/smlnj/isabelle/source.html
Dave MacQueen <dbm@research.bell-labs.com>