docs: private void add(FieldFilter filter) {
          switch(operator) {
          case NOT:
              filters.add(new NegationFieldFilter(filter));
              operator = Operator.NONE;
              break;
          case OR:
              filters.set(filters.size() - 1, 
                          new OrFieldFilter(filters.get(filters.size() - 1), filter));
              operator = Operator.NONE;
              break;
          default:
              filters.add(filter);
              break;
          }
      }