code: func copyWithLeftPad(dest, src []byte) {
  	numPaddingBytes := len(dest) - len(src)
  	for i := 0; i < numPaddingBytes; i++ {
  		dest[i] = 0
  	}
  	copy(dest[numPaddingBytes:], src)
  }
docs: // copyWithLeftPad copies src to the end of dest, padding with zero bytes as
  // needed.