code: func paddedAppend(size uint, dst, src []byte) []byte {
  	for i := 0; i < int(size)-len(src); i++ {
  		dst = append(dst, 0)
  	}
  	return append(dst, src...)
  }
docs: // paddedAppend appends the src byte slice to dst, returning the new slice.
  // If the length of the source is smaller than the passed size, leading zero
  // bytes are appended to the dst slice before appending src.