Initialise a collection to an extension of another collection

gistfile1.cs

        private static readonly IEnumerable<Foo> BaseCollectionOfFoo = new List<Foo>
                                                                       {
                                                                           new Foo(1),
                                                                           new Foo(2),
                                                                           new Foo(3)
                                                                       };

        private static readonly IEnumerable<Foo> AllFoo = new List<Foo>(BaseCollectionOfFoo)
                                                                       {
                                                                           new Foo(4)
                                                                       };