erikrozendaal/scala — comment on commit b9699f999da24f72dca65ecfb066b0ac3151f2b5
File: src/library/scala/collection/immutable/RedBlack.scala line 19
By erikrozendaal:

Mainly to make it possible to further improve the implementation (performance) without causing more pain later, such as replacing the Empty singleton instance with null and relying less on polymorphism.

Right now seems a good time because RedBlack has changed so much anyway that it is definitely no longer source (or binary) compatible.