code: final int getIndexValue() {
          if (anteContextLength == pattern.length()) {
              // A pattern with just ante context {such as foo)>bar} can
              // match any key.
              return -1;
          }
          int c = UTF16.charAt(pattern, anteContextLength);
          return data.lookupMatcher(c) == null ? (c & 0xFF) : -1;
      }
docs: Internal method.  Returns 8-bit index value for this rule.
  This is the low byte of the first character of the key,
  unless the first character of the key is a set.  If it's a
  set, or otherwise can match multiple keys, the index value is -1.