var (
		cntNewline int64
		buf        bytes.Buffer
		blk        block
	)

	// feedTokens copies data in blocks from r into buf until there are
	// at least cnt newlines in buf. It will not read more blocks than needed.
	feedTokens := func(n int64) error {
		for cntNewline < n {
			if _, err := mustReadFull(r, blk[:]); err != nil {
				return err
			}
			buf.Write(blk[:])