code: func NewWrapWriter(w io.WriterAt, off int64, wrapAt int64) *WrapWriter {
  	return &WrapWriter{
  		&wrapper{
  			doat:   w.WriteAt,
  			off:    (off % wrapAt),
  			wrapAt: wrapAt,
  		},
  	}
  }
docs: // NewWrapWriter creates a WrapWriter starting at offset off, and wrapping at offset wrapAt.