code: func (c *Client) bundlerHandler(iface interface{}) {
  	entries := iface.([]*batchEntry)
  	if len(entries) == 0 {
  		return
  	}
  
  	ctx, opts := entries[0].ctx, entries[0].opts
  
  	c.outstanding.Add(1)
  	go func() {
  		defer c.outstanding.Done()
  		c.sendBundle(ctx, entries, opts...)
  	}()
  }
docs: // bundleHandler is called when a bundle threshold has been met.
  //
  // This is a bundler.Bundler handler function. "iface" is []*batchEntry{}, a
  // slice of the prototype passed into NewBundler.
  //
  // Note that "iface" is owned by this handler; the Bundler allocates a new
  // slice after each bundle dispatch. Therefore, retention and mutation are safe.