code: public void setLemmaString(String lemmaString) {
      if (!StringTools.isEmpty(lemmaString)) {
        lemma = lemmaString;
        staticLemma = true;
        postagRegexp = true;
        if (posTag != null) {
          pPosRegexMatch = Pattern.compile(posTag);
        }
      }
    }
docs: Sets a base form (lemma) that will be formatted, or synthesized, using the
  specified POS regular expressions.
  @param lemmaString String that specifies the base form.