code: func (c *Client) GetCFilterAsync(blockHash *chainhash.Hash,
  	filterType wire.FilterType) FutureGetCFilterResult {
  	hash := ""
  	if blockHash != nil {
  		hash = blockHash.String()
  	}
  
  	cmd := btcjson.NewGetCFilterCmd(hash, filterType)
  	return c.sendCmd(cmd)
  }
docs: // GetCFilterAsync 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 GetCFilter for the blocking version and more details.