code: func NewSectionReadSeeker(base io.ReadSeeker, off, size int64) (ret ReadSeekContexter) {
  	ret = &sectionReadSeeker{
  		base: base,
  		off:  off,
  		size: size,
  	}
  	seekOff, err := ret.Seek(0, io.SeekStart)
  	if err != nil {
  		panic(err)
  	}
  	if seekOff != 0 {
  		panic(seekOff)
  	}
  	return
  }
docs: // Returns a ReadSeeker on a section of another ReadSeeker.