code: func newSyntheticByteStream(size int64, factory func(int64) byte) io.ReadSeeker {
  	return &syntheticByteStream{
  		size:    size,
  		factory: factory,
  	}
  }
docs: // newSyntheticByteStream returns a new stream of bytes of a specific size,
  // given a factory function for generating the byte at a given offset.