code: func (id *ULID) SetEntropy(e []byte) error {
  	if len(e) != 10 {
  		return ErrDataSize
  	}
  
  	copy((*id)[6:], e)
  	return nil
  }
docs: // SetEntropy sets the ULID entropy to the passed byte slice.
  // ErrDataSize is returned if len(e) != 10.