diff --git a/config/file.go b/config/file.go
index da1d647..abd3232 100644
--- a/config/file.go
+++ b/config/file.go
@@ -13,6 +13,8 @@ type line any
 type comment string
 type entry struct { key string; value Value }
 
+var _ io.WriterTo = new(File)
+
 // File represents a config file. It preserves the order of the lines, as well
 // as blank lines and comments.
 type File struct {
@@ -251,8 +253,8 @@ func (this *File) Diff (other *File) map[string] struct { } {
 }
 
 // WriteTo writes the data in this file to an io.Writer.
-func (file *File) WriteTo (writer io.Writer) (n int64, err error) {
-	for _, lin := range file.lines {
+func (this *File) WriteTo (writer io.Writer) (n int64, err error) {
+	for _, lin := range this.lines {
 		nint := 0
 		switch lin := lin.(type) {
 		case comment: