}
	sort.Strings(fs)
	return
}

var parsedFileCache = make(map[string]*ast.File)

func (w *Walker) parseFile(dir, file string) (*ast.File, error) {
	filename := filepath.Join(dir, file)
	if f := parsedFileCache[filename]; f != nil {
		return f, nil
	}

	f, err := parser.ParseFile(fset, filename, nil, parser.ParseComments)
	if err != nil {
		return nil, err
	}
	parsedFileCache[filename] = f

import (
	"fmt"
	"go/ast"
	"go/format"
	"go/parser"
	"go/scanner"
	"go/token"
	"os"
	"strings"
)

func parse(name string, src []byte, flags parser.Mode) *ast.File {
	ast1, err := parser.ParseFile(fset, name, src, flags)
	if err != nil {
		if list, ok := err.(scanner.ErrorList); ok {
			// If err is a scanner.ErrorList, its String will print just
			// the first error and then (+n more errors).

pkg go/parser, const Trace Mode
pkg go/parser, func ParseDir(*token.FileSet, string, func(os.FileInfo) bool, Mode) (map[string]*ast.Package, error)
pkg go/parser, func ParseExpr(string) (ast.Expr, error)
pkg go/parser, func ParseFile(*token.FileSet, string, interface{}, Mode) (*ast.File, error)
pkg go/parser, type Mode uint
pkg go/printer, const RawFormat Mode
pkg go/printer, const SourcePos Mode
pkg go/printer, const TabIndent Mode