code: func Sum224(data []byte) (sum224 [Size224]byte) {
  	var d digest
  	d.is224 = true
  	d.Reset()
  	d.Write(data)
  	sum := d.checkSum()
  	copy(sum224[:], sum[:Size224])
  	return
  }
docs: // Sum224 returns the SHA224 checksum of the data.