Attaching the problem file...

Attaching the example domain that caused the problem. The issue is with the 
grounding of the drive action (a passenger should not be allowed to drive a 
taxi), and most generated plans will not validate because of this bad 
grounding. I left the other actions in so the problem remained solvable.

Fixed.

Christian, I'd like to attach your example domain/problem file (in the version
with the rewritten type system) as a test case for this, but I'm not 100% sure
if it's OK to share them publicly. If it is, it would be great if you could
attach them, or just let me know and I can do it.

The handling of atoms like "P(?x, ?y, ?x)" where the same variable occurs
multiple times in one atom is broken.