type 3 sums of squares

[Jason Stover]

I am getting around to coding type 3 sums of squares. I think
the easiest approach is to put a function in linreg.c whose call
would be something like

ssq = linreg_type3_ssq (cov, vars, n_vars, l);

Where l is a linreg struct, and the other args are what they seem to
be.

My question is: Should this be something in glm.c, so as to avoid use
of linreg.c by glm.c? Or should glm.c depend on linreg.c?

I think using linreg.c would hide a lot details about computation of
type 3 sums of squares. On the other hand, it would necessitate
creating a linreg struct. But then again, having a linreg struct may
be useful if PSPP ever gets to a point where one procedure can use
data structures from previously called procedures. 

-Jason