var prev span
	for _, s := range parseSpans(lines) {
		var b Block
		switch s.kind {
		default:
			panic("go/doc/comment: internal error: unknown span kind")
		case spanList:
			b = d.list(lines[s.start:s.end], prev.end < s.start)
		case spanCode:
			b = d.code(lines[s.start:s.end])
		case spanOldHeading:
			b = d.oldHeading(lines[s.start])
		case spanHeading: