code: func NewWrapReader(r io.ReaderAt, off int64, wrapAt int64) *WrapReader {
  	return &WrapReader{
  		&wrapper{
  			doat:   r.ReadAt,
  			off:    (off % wrapAt),
  			wrapAt: wrapAt,
  		},
  	}
  }
docs: // NewWrapReader creates a WrapReader starting at offset off, and wrapping at offset wrapAt.