code: private static <K,V> boolean colorOf(TreeMapEntry<K,V> p) {
          return (p == null ? BLACK : p.color);
      }
docs: Balancing operations.
  
  Implementations of rebalancings during insertion and deletion are
  slightly different than the CLR version.  Rather than using dummy
  nilnodes, we use a set of accessors that deal properly with null.  They
  are used to avoid messiness surrounding nullness checks in the main
  algorithms.