code: func (c *Client) ListSinceBlockAsync(blockHash *chainhash.Hash) FutureListSinceBlockResult {
  	var hash *string
  	if blockHash != nil {
  		hash = btcjson.String(blockHash.String())
  	}
  
  	cmd := btcjson.NewListSinceBlockCmd(hash, nil, nil)
  	return c.sendCmd(cmd)
  }
docs: // ListSinceBlockAsync returns an instance of a type that can be used to get
  // the result of the RPC at some future time by invoking the Receive function on
  // the returned instance.
  //
  // See ListSinceBlock for the blocking version and more details.