public Query add(Map<String, ? extends Iterable<String>> values) {
            Query query = this;
            for (Map.Entry<String, ? extends Iterable<String>> entry : values.entrySet())
                for (String value : entry.getValue())
                    query = value == null ? query.add(entry.getKey())
                                          : query.add(entry.getKey(), value);

            return query;
        }
Returns a copy of this with all given mappings appended to this. {@code null} values, but not lists of values, are allowed.
vespajlib/src/main/java/ai/vespa/http/HttpURL.java