The line in pks
if (z == 0.) return 0.;
should be replaced by
if (z < 0.042) return 0.;
This fixes a potential underflow in the exponential, two lines later. Thanks to user tbegg for pointing this out.

Hi,

KSdist::invqks warning "The local variable yp is never read".

Is yp necessary?

Yes, you're right. It's a debug fossil. Remove the declaration of yp, and the line
yp = y;
Thanks for pointing this out.