diff --git a/config/file.go b/config/file.go
index a99a4a1..2fc10af 100644
--- a/config/file.go
+++ b/config/file.go
@@ -143,6 +143,14 @@ func ParseValue (str string) (Value, error) {
 	}
 }
 
+// Has returns whether the key exists. If the key is invalid, it returns false,
+// ErrMalformedKey.
+func (this *File) Has (key string) (bool, error) {
+	if !KeyValid(key) { return false, ErrMalformedKey }
+	_, ok := this.keys[key]
+	return ok, nil
+}
+
 // Get gets the keyed value. If the value is unspecified, it returns nil,
 // ErrNonexistentEntry. If the key is invalid, it returns nil, ErrMalformedKey.
 func (this *File) Get (key string) (Value, error) {
@@ -186,7 +194,7 @@ func (this *File) Reset (key string) error {
 	return nil
 }
 
-// Map returns a map of keys to values.
+// Map creates and returns a map of keys to values.
 func (this *File) Map () map[string] Value {
 	mp := make(map[string] Value)
 	for key, index := range this.keys {
@@ -197,6 +205,33 @@ func (this *File) Map () map[string] Value {
 	return mp
 }
 
+// Diff returns a set of keys that are different from the other file.
+func (this *File) Diff (other *File) map[string] struct { } {
+	diff := make(map[string] struct { })
+	
+	// - keys only we have
+	// - keys we both have, but are different
+	for key, index := range this.keys {
+		otherIndex, ok := other.keys[key]
+		if !ok {
+			diff[key] = struct { } { }
+			continue
+		}
+		if !this.lines[index].(entry).value.Equals(other.lines[otherIndex].(entry).value) {
+			diff[key] = struct { } { }
+		}
+	}
+
+	// - keys only they have
+	for key := range other.keys {
+		if _, has := this.keys[key]; !has {
+			diff[key] = struct { } { }
+		}
+	}
+
+	return diff
+}
+
 // 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 {
diff --git a/config/file_test.go b/config/file_test.go
index 0d081c4..8a696c7 100644
--- a/config/file_test.go
+++ b/config/file_test.go
@@ -1,6 +1,7 @@
 package config
 
 import "math"
+import "maps"
 import "strings"
 import "testing"
 
@@ -103,6 +104,51 @@ key1=7
 `)
 }
 
+func TestDiffNone (test *testing.T) {
+	str := `
+thing = something
+
+otherThing  = otherValue
+# comment
+otherThing = true
+otherThing = 234
+
+yetAnotherThing = 0.23498
+`
+	file1 := parseFileString(test, str)
+	file2 := parseFileString(test, str)
+	diff := file1.Diff(file2)
+	if len(diff) != 0 {
+		test.Fatalf("diff not empty:\n%v", diff)
+	}
+}
+
+func TestDiff (test *testing.T) {
+	file1 := parseFileString(test,
+`key4=0
+key1=value1
+# comment
+key2=34`)
+	file2 := parseFileString(test,
+`key1=value2
+key2=34
+# comment
+
+key3=0.2`)
+	diff := file1.Diff(file2)
+	correct := map[string] struct { } {
+		"key1": struct { } { },
+		"key3": struct { } { },
+		"key4": struct { } { },
+	}
+	if !maps.Equal(diff, correct) {
+		test.Error("diffs do not match")
+		test.Errorf("EXPECTED:\n%v", correct)
+		test.Errorf("GOT:\n%v", diff)
+		test.Fail()
+	}
+}
+
 func testParseEntry (test *testing.T, str string, key string, value Value) {
 	ent, err := parseEntry(str)
 	if err != nil { test.Fatal(err) }