Companion

Base class for spliterators. Iteration starts at a given origin and continues up to but not including a given fence (or null for end). At top-level, for ascending cases, the first split uses the root as left-fence/right-origin. From there, right-hand splits replace the current fence with its left child, also serving as origin for the split-off spliterator. Left-hands are symmetric. Descending versions place the origin at the end and invert ascending split rules. This base class is non-committal about directionality, or whether the top-level spliterator covers the whole tree. This means that the actual split mechanics are located in subclasses. Some of the subclass trySplit methods are identical (except for return types), but not nicely factorable.

Currently, we support Spliterator-based versions only for the full map, in either plain of descending form, otherwise relying on defaults because size estimation for submaps would dominate costs. The type tests needed to check these for key views are not very nice but avoid disrupting existing class structures. Callers must use plain default spliterators if this returns null.

Test two values for equality. Differs from o1.equals(o2) only in that it copes with null o1 properly.