diff --git a/fs.go b/fs.go
new file mode 100644
index 0000000..a699fac
--- /dev/null
+++ b/fs.go
@@ -0,0 +1,169 @@
+package nasin
+
+import "io"
+import "os"
+import "io/fs"
+import "strings"
+import "path/filepath"
+
+// FS is an io.FS with extended capabilities that include writing to the
+// underlying filesystem.
+type FS interface {
+	fs.FS
+	fs.ReadDirFS
+	fs.ReadFileFS
+
+	// OpenFile is like os.OpenFile.
+	OpenFile (name string, flag int, perm os.FileMode) (FileWriter, error)
+
+	// Remove removes the named file or (empty) directory.
+	Remove (name string) error
+	
+	// RemoveAll removes name and any children it contains.
+	RemoveAll (name string) error
+	
+	// Rename renames (moves) oldname to newname.
+	Rename (oldname, newname string) error
+}
+
+// FileWriter is a writable version of fs.File.
+type FileWriter interface {
+	fs.File
+	io.Writer
+}
+
+type writeFs struct {
+	path string
+}
+
+func pathErr (op, path string, err error) error {
+	return &fs.PathError {
+		Op:   op,
+		Path: path,
+		Err:  err,
+	}
+}
+
+func (this writeFs) subPath (name string) (string, error) {
+	if !fs.ValidPath(name)         { return "", fs.ErrInvalid }
+	if strings.Contains(name, "/") { return "", fs.ErrInvalid }
+	return filepath.Join(this.path, name), nil
+}
+
+// Open opens the named file.
+func (this writeFs) Open (name string) (fs.File, error) {
+	path, err := this.subPath(name)
+	if err != nil {
+		return nil, pathErr("open", name, err)
+	}
+
+	return os.Open(path)
+}
+
+// Create creates or truncates the named file.
+func (this writeFs) Create (name string) (FileWriter, error) {
+	path, err := this.subPath(name)
+	if err != nil {
+		return nil, pathErr("create", name, err)
+	}
+
+	return os.Create(path)
+}
+
+// OpenFile is the generalized open call; most users will use Open or Create
+// instead.
+func (this writeFs) OpenFile (
+	name string,
+	flag int,
+	perm os.FileMode,
+) (
+	FileWriter,
+	error,
+) {
+	path, err := this.subPath(name)
+	if err != nil {
+		return nil, pathErr("open", name, err)
+	}
+
+	return os.OpenFile(path, flag, perm)
+}
+
+// ReadDir reads the named directory and returns a list of directory entries
+// sorted by filename.
+func (this writeFs) ReadDir (name string) ([]fs.DirEntry, error) {
+	path, err := this.subPath(name)
+	if err != nil {
+		return nil, pathErr("readdir", name, err)
+	}
+
+	return os.ReadDir(path)
+}
+
+// ReadFile reads the named file and returns its contents.
+// A successful call returns a nil error, not io.EOF.
+// (Because ReadFile reads the whole file, the expected EOF
+// from the final Read is not treated as an error to be reported.)
+//
+// The caller is permitted to modify the returned byte slice.
+func (this writeFs) ReadFile (name string) ([]byte, error) {
+	path, err := this.subPath(name)
+	if err != nil {
+		return nil, pathErr("readfile", name, err)
+	}
+
+	return os.ReadFile(path)
+}
+
+// WriteFile writes data to the named file, creating it if necessary.
+func (this writeFs) WriteFile (name string, data []byte, perm os.FileMode) error {
+	path, err := this.subPath(name)
+	if err != nil {
+		return pathErr("writefile", name, err)
+	}
+
+	return os.WriteFile(path, data, perm)
+}
+
+// Stat returns a FileInfo describing the file.
+func (this writeFs) Stat (name string) (fs.FileInfo, error) {
+	path, err := this.subPath(name)
+	if err != nil {
+		return nil, pathErr("stat", name, err)
+	}
+
+	return os.Stat(path)
+}
+
+// Remove removes the named file or (empty) directory.
+func (this writeFs) Remove (name string) error {
+	path, err := this.subPath(name)
+	if err != nil {
+		return pathErr("remove", name, err)
+	}
+
+	return os.Remove(path)
+}
+
+// RemoveAll removes name and any children it contains.
+func (this writeFs) RemoveAll (name string) error {
+	path, err := this.subPath(name)
+	if err != nil {
+		return pathErr("removeall", name, err)
+	}
+
+	return os.RemoveAll(path)
+}
+
+// Rename renames (moves) oldname to newname.
+func (this writeFs) Rename (oldname, newname string) error {
+	oldpath, err := this.subPath(oldname)
+	if err != nil {
+		return pathErr("rename", oldname, err)
+	}
+	newpath, err := this.subPath(newname)
+	if err != nil {
+		return pathErr("rename", newname, err)
+	}
+
+	return os.Rename(oldpath, newpath)
+}