code: func (r *BodyWithCallback) Seek(offset int64, whence int) (int64, error) {
  	switch whence {
  	case io.SeekStart:
  		r.readSize = offset
  	case io.SeekCurrent:
  		r.readSize += offset
  	case io.SeekEnd:
  		r.readSize = r.totalSize + offset
  	}
  
  	return r.ReadSeekCloser.Seek(offset, whence)
  }
docs: // Seek wraps the underlying Seeker's "Seek" method, updating the number of
  // bytes that have been consumed by this reader.