docs: public static Tuple<String, String> substring(String strColumn, int pos, int len, String valueName) {
          String name = "substring_" + random();
          if (valueName == null) {
              return new Tuple(name, "def " + name + " = doc['" + strColumn + "'].value.substring(" + pos + "," + len + ")");
          } else {
              return new Tuple(name, strColumn + ";def " + name + " = " + valueName + ".substring(" + pos + "," + len + ")");
          }
  
      }