code: V getValidValue() {
              Object v = value;
              if (v == sentinel() || v == BASE_HEADER)
                  return null;
              @SuppressWarnings("unchecked") V vv = (V)v;
              return vv;
          }
docs: Returns value if this node contains a valid key-value pair,
  else null.
  @return this node's value if it isn't a marker or header or
  is deleted, else null