code: func (r *reader) nextByte() byte {
  	r.pos += 1
  	if r.pos > r.size {
  		_panicf("nextByte: insufficient buffer (%d of %d)", r.pos, r.size)
  	}
  	return r.buf[r.pos-1]
  }
docs: // nextByte reads the next byte from the buffer