- commonly used, only depends on routines defined in UList
(don't need the rest of ListOps for it).
ENH: implement boolList::operator() const
- allows use as a predicate functor, as per bitSet and labelHashSet
GIT: combine SubList, UList into List directory (intertwined concepts)
STYLE: default initialize DynamicList instead of with size 0
- the NullObject singleton can also be cast to a bitSet
(sufficient size and bit-pattern). Useful for places that
need to hold a reference on construction