protected function execute(InputInterface $input, OutputInterface $output)
 {
     $expectations = [$input->getArgument('expectation')];
     if ($expectations === [NULL]) {
         $expectations = ExpectationHelper::getAll();
     }
     foreach ($expectations as $expectation) {
         if (!ExpectationHelper::has($expectation)) {
             $output->writeln("<error>Expectation '{$expectation}' not found</error>");
             continue;
         }
         $output->writeln("<info>{$expectation}:</info>");
         $expectation = ExpectationHelper::get($expectation);
         if (method_exists($expectation, "describe")) {
             $output->writeln($expectation::describe());
         }
         $output->writeln("");
     }
 }