Python derived predicates cannot use boolean operations

Predicates defined in py/pack.py in pure Python cannot be combined using boolean operations:

# should produce trapezoid as box - halfSpace
ab = pack.inAlignedBox((0,0,0),(30,10,20))
hs = pack.inHalfSpace((30,0,0),(-1,0,-2))
pred = ab - hs
spheres = pack.randomDensePack(pred,radius=1,spheresInCell=300)

results in:

TypeError: No registered converter was able to extract a C++ reference to type Predicate from this Python object of type inHalfSpace