code: func (wpr *Wrapper) ReadAt(p []byte, off int64) (n int, err error) {
  	wrap := NewWrapReader(wpr.rwa, wpr.O+off, wpr.N)
  	r := io.LimitReader(wrap, wpr.L-off)
  	return r.Read(p)
  }
docs: // ReadAt reads from the current offset+off into p, wrapping at Cap()