code: private boolean haveExperimentalLanguagePrefix() {
          if (id.length > 2) {
              char c = id[1];
              if (c == HYPHEN || c == UNDERSCORE) {
                  c = id[0];
                  return c == 'x' || c == 'X' || c == 'i' || c == 'I';
              }
          }
          return false;
      }
docs: Returns true if the start of the buffer has an experimental or private language
  prefix, the pattern '[ixIX][-_].' shows the syntax checked.