PFix is a partially defined fixpoint combinator.
The above "fixpoint combinator" means, PFix(h).toPartialFunction = h(PFix(h).toPartialFunction) holds for any h.
PFix is a partially defined fixpoint combinator.
The above "fixpoint combinator" means, PFix(h).toPartialFunction = h(PFix(h).toPartialFunction) holds for any h.
This implementation is efficient for combining multiple instances via orElse method.