f.Comments = ast1.Comments
	f.AST = ast2
}

// Like ast.CommentGroup's Text method but preserves
// leading blank lines, so that line numbers line up.
func commentText(g *ast.CommentGroup) string {
	pieces := make([]string, 0, len(g.List))
	for _, com := range g.List {
		c := com.Text
		// Remove comment markers.
		// The parser has given us exactly the comment text.
		switch c[1] {