code: func (c *ContainerIO) AddOutput(name string, stdout, stderr io.WriteCloser) (io.WriteCloser, io.WriteCloser) {
  	var oldStdout, oldStderr io.WriteCloser
  	if stdout != nil {
  		key := streamKey(c.id, name, Stdout)
  		oldStdout = c.stdoutGroup.Get(key)
  		c.stdoutGroup.Add(key, stdout)
  	}
  	if stderr != nil {
  		key := streamKey(c.id, name, Stderr)
  		oldStderr = c.stderrGroup.Get(key)
  		c.stderrGroup.Add(key, stderr)
  	}
  	return oldStdout, oldStderr
  }
docs: // AddOutput adds new write closers to the container stream, and returns existing
  // write closers if there are any.