code: AbstractMap.SimpleImmutableEntry<K,V> createSnapshot() {
              Object v = value;
              if (v == null || v == sentinel() || v == BASE_HEADER)
                  return null;
              @SuppressWarnings("unchecked") V vv = (V)v;
              return new AbstractMap.SimpleImmutableEntry<K,V>(key, vv);
          }
docs: Creates and returns a new SimpleImmutableEntry holding current
  mapping if this node holds a valid value, else null.
  @return new entry or null