code: public void addFieldValue(String fieldName, String value) {
          Utils.require(!Utils.isEmpty(fieldName), "fieldName");
          
          if (fieldName.charAt(0) == '_') {
              setSystemField(fieldName, value);
          } else {
              List<String> currValues = m_valueMap.get(fieldName);
              if (currValues == null) {
                  currValues = new ArrayList<>();
                  m_valueMap.put(fieldName, currValues);
              }
              currValues.add(value);
          }
      }
docs: Add the given value to the field with the given name. For a system field, its
  existing value, if any, is replaced. If a null or empty field is added for an
  SV scalar, the field is nullified.
  
  @param fieldName Name of a field.
  @param value     Value to add to field. Ignored if null or empty.