diff --git a/backend/walker.go b/backend/walker.go
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+++ b/backend/walker.go
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+// Copyright 2020 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package backend
+
+import (
+	"io/fs"
+	"path"
+)
+
+// WalkDirFunc is the type of the function called by [WalkDir] to visit
+// each file or directory.
+//
+// The path argument contains the argument to [WalkDir] as a prefix.
+// That is, if WalkDir is called with root argument "dir" and finds a file
+// named "a" in that directory, the walk function will be called with
+// argument "dir/a".
+//
+// The d argument is the [DirEntry] for the named path.
+//
+// The error result returned by the function controls how [WalkDir]
+// continues. If the function returns the special value [SkipDir], WalkDir
+// skips the current directory (path if d.IsDir() is true, otherwise
+// path's parent directory). If the function returns the special value
+// [SkipAll], WalkDir skips all remaining files and directories. Otherwise,
+// if the function returns a non-nil error, WalkDir stops entirely and
+// returns that error.
+//
+// The err argument reports an error related to path, signaling that
+// [WalkDir] will not walk into that directory. The function can decide how
+// to handle that error; as described earlier, returning the error will
+// cause WalkDir to stop walking the entire tree.
+//
+// [WalkDir] calls the function with a non-nil err argument in two cases.
+//
+// First, if the initial [Stat] on the root directory fails, WalkDir
+// calls the function with path set to root, d set to nil, and err set to
+// the error from [fs.Stat].
+//
+// Second, if a directory's ReadDir method (see [ReadDirFile]) fails, WalkDir calls the
+// function with path set to the directory's path, d set to an
+// [DirEntry] describing the directory, and err set to the error from
+// ReadDir. In this second case, the function is called twice with the
+// path of the directory: the first call is before the directory read is
+// attempted and has err set to nil, giving the function a chance to
+// return [SkipDir] or [SkipAll] and avoid the ReadDir entirely. The second call
+// is after a failed ReadDir and reports the error from ReadDir.
+// (If ReadDir succeeds, there is no second call.)
+//
+// The differences between WalkDirFunc compared to [path/filepath.WalkFunc] are:
+//
+//   - The second argument has type [DirEntry] instead of [FileInfo].
+//   - The function is called before reading a directory, to allow [SkipDir]
+//     or [SkipAll] to bypass the directory read entirely or skip all remaining
+//     files and directories respectively.
+//   - If a directory read fails, the function is called a second time
+//     for that directory to report the error.
+type WalkDirFunc func(path string, d fs.DirEntry, err error) error
+
+// walkDir recursively descends path, calling walkDirFn.
+func walkDir(fsys fs.FS, name string, d fs.DirEntry, walkDirFn WalkDirFunc) error {
+	if err := walkDirFn(name, d, nil); err != nil || !d.IsDir() {
+		if err == fs.SkipDir && d.IsDir() {
+			// Successfully skipped directory.
+			err = nil
+		}
+		// return errors.New("hi " + name)
+		 return err
+	}
+
+	dirs, err := fs.ReadDir(fsys, name)
+	if err != nil {
+		// Second call, to report ReadDir error.
+		err = walkDirFn(name, d, err)
+		if err != nil {
+			if err == fs.SkipDir && d.IsDir() {
+				err = nil
+			}
+			return err
+		}
+	}
+
+	for _, d1 := range dirs {
+		name1 := path.Join(name, d1.Name())
+		if err := walkDir(fsys, name1, d1, walkDirFn); err != nil {
+			if err == fs.SkipDir {
+				break
+			}
+			return err
+		}
+	}
+	return nil
+}
+
+// WalkDir walks the file tree rooted at root, calling fn for each file or
+// directory in the tree, including root.
+//
+// All errors that arise visiting files and directories are filtered by fn:
+// see the [fs.WalkDirFunc] documentation for details.
+//
+// The files are walked in lexical order, which makes the output deterministic
+// but requires WalkDir to read an entire directory into memory before proceeding
+// to walk that directory.
+//
+// WalkDir does not follow symbolic links found in directories,
+// but if root itself is a symbolic link, its target will be walked.
+func WalkDir(fsys fs.FS, root string, fn WalkDirFunc) error {
+	info, err := fs.Stat(fsys, root)
+	if err != nil {
+		err = fn(root, nil, err)
+	} else {
+		err = walkDir(fsys, root, fs.FileInfoToDirEntry(info), fn)
+	}
+	if err == fs.SkipDir || err == fs.SkipAll {
+		return nil
+	}
+	return err
+}