Changes to make egcs work

["Asger K. Alstrup Nielsen" <alstrup diku dk>]

Hi!

I changed a few trivial things to suit egcs better,
and now it seems to work on my system, at least.
(I.e. it compiles.  I have no test code for the
HoleSet yet.)

So it compiles on Win32 and with the version I have:
egcs-2.90.27 980315 (egcs-1.0.2 release)

If you have a different compiler, please give it
a spin and tell me if it fails.  If not, I'll keep
the code as it is, because I think it's a beautiful
general solution to a relatively hard problem:

Take an existing DataSet derivate that does not support
missing data, and *extend* it into one that does,
and that still support the extra functionality that the
original had.  Pretty neat.

Greets,

Asger