code: protected function writeProgress($progress)
      {
          // Record progress so it can be written later instead of at start of line.
          $this->progress = !$this->flawless && $progress === '.'
              // If a previous error or exception occurred, replace '.' with red '!'.
              ? $this->formatWithColor('fg-red', '!')
              : $progress
          ;
  
          ++$this->numTestsRun;
      }
docs: Writes progress to the output buffer.
  
  @param string $progress