+signup +login

frontend/pshared/lib/models/reference.dart

@@ -0,0 +1,22 @@
+abstract class Reference {
+  String get ref;
+}
+
+class _ReferenceImp implements Reference {
+  @override
+  final String ref;
+
+  const _ReferenceImp({
+    required this.ref,
+  });
+}
+
+Reference newReference({required String ref}) => _ReferenceImp(ref: ref);
+
+extension ReferenceCopier on Reference {
+  Reference copyWith({
+    String? ref,
+  }) => newReference(
+    ref: ref ?? this.ref,
+  );
+}