feat(tonality): ✨ add isTheoretical getter

lib/src/tonality/tonality.dart

@@ -1,11 +1,14 @@
 part of '../../music_notes.dart';
 
-/// A musical tonality.
+/// A musical tonality, also known as key.
+///
+/// See [Key (music)](https://en.wikipedia.org/wiki/Key_(music)).
 ///
 /// ---
 /// See also:
 /// * [Note].
 /// * [Mode].
+/// * [KeySignature].
 @immutable
 final class Tonality implements Comparable<Tonality> {
   /// The tonal center representing this [Tonality].
@@ -56,6 +59,19 @@ final class Tonality implements Comparable<Tonality> {
         KeySignature.empty.tonality(mode)!.note.fifthsDistanceWith(note),
       );
 
+  /// Whether this [Tonality] is theoretical, whose [keySignature] would have
+  /// at least one [Accidental.doubleFlat] or [Accidental.doubleSharp].
+  ///
+  /// See [Theoretical key](https://en.wikipedia.org/wiki/Theoretical_key).
+  ///
+  /// Example:
+  /// ```dart
+  /// Note.e.major.isTheoretical == false
+  /// Note.g.sharp.major.isTheoretical == true
+  /// Note.f.flat.minor.isTheoretical == true
+  /// ```
+  bool get isTheoretical => (keySignature.distance?.abs() ?? 0) > 7;
+
   /// Returns the scale notes of this [Tonality].
   ///
   /// Example:

test/src/tonality/tonality_test.dart

@@ -78,6 +78,24 @@ void main() {
       });
     });
 
+    group('.isTheoretical', () {
+      test('returns whether this Tonality is theoretical', () {
+        expect(Note.c.flat.major.isTheoretical, false);
+        expect(Note.a.flat.minor.isTheoretical, false);
+        expect(Note.c.minor.isTheoretical, false);
+        expect(Note.e.major.isTheoretical, false);
+        expect(Note.c.sharp.major.isTheoretical, false);
+        expect(Note.a.sharp.minor.isTheoretical, false);
+
+        expect(Note.f.flat.major.isTheoretical, true);
+        expect(Note.d.flat.minor.isTheoretical, true);
+        expect(Note.c.sharp.sharp.minor.isTheoretical, true);
+        expect(Note.a.flat.flat.major.isTheoretical, true);
+        expect(Note.g.sharp.major.isTheoretical, true);
+        expect(Note.e.sharp.minor.isTheoretical, true);
+      });
+    });
+
     group('.scale', () {
       test('returns the scale notes of this Tonality', () {
         expect(