public function requireOneOf(array $fieldnames, $count = 1) {
        $result = $this->addValidator('*', function ($data, Validation $validation) use ($fieldnames, $count) {
            $hasCount = 0;
            $flattened = [];

            foreach ($fieldnames as $name) {
                $flattened = array_merge($flattened, (array)$name);

                if (is_array($name)) {
                    // This is an array of required names. They all must match.
                    $hasCountInner = 0;
                    foreach ($name as $nameInner) {
                        if (isset($data[$nameInner]) && $data[$nameInner]) {
                            $hasCountInner++;
                        } else {
                            break;
                        }
                    }
                    if ($hasCountInner >= count($name)) {
                        $hasCount++;
                    }
                } elseif (isset($data[$name]) && $data[$name]) {
                    $hasCount++;
                }

                if ($hasCount >= $count) {
                    return true;
                }
            }

            $messageFields = array_map(function ($v) {
                if (is_array($v)) {
                    return '('.implode(', ', $v).')';
                }
                return $v;
            }, $fieldnames);

            if ($count === 1) {
                $message = sprintft('One of %s are required.', implode(', ', $messageFields));
            } else {
                $message = sprintft('%1$s of %2$s are required.', $count, implode(', ', $messageFields));
            }

            $validation->addError('missing_field', $flattened, [
                'message' => $message
            ]);
            return false;
        });

        return $result;
    }