public function fail($label, $computed, $test, $expected) {
    if ($this->output && !$this->output->isQuiet()) {
      $this->output->write("<fg=red>F</fg=red>");
    }
    $this->failed += 1;
    
    $this->failures[] = array(
      'label'    => $label,
      'computed' => $computed,
      'test'     => $test,
      'expected' => $expected
    );
  }