code: public final char getSurrogateValue(char lead, char trail)
      {
          int offset = getSurrogateOffset(lead, trail);
          if (offset > 0) {
              return m_data_[offset];
          }
          return m_initialValue_;
      }
docs: Get the value associated with a pair of surrogates.
  @param lead a lead surrogate
  @param trail a trail surrogate