code: private void parseFieldAdd(String fieldName, UNode addNode) {
          Set<String> addValueSet = new LinkedHashSet<>();
          for (UNode valueNode : addNode.getMemberList()) {
              Utils.require(valueNode.isValue() && valueNode.getName().equals("value"),
                            "Value expected for 'add' element: " + valueNode);
              addValueSet.add(valueNode.getValue());
          }
          addFieldValues(fieldName, addValueSet);
      }
docs: Parse an "add" update to a field.