Just as it is possible to define custom sequential collections without
defining their builders, it is possible to define parallel collections without
defining their combiners. The consequence of not having a combiner is that
transformer methods (e.g.
filter, …) will by
default return a standard collection type which is nearest in the hierarchy.
For example, ranges do not have builders, so mapping elements of a range
creates a vector.
This is a companion discussion topic for the original entry at http://docs.scala-lang.org//overviews/parallel-collections/custom-parallel-collections