public void WithInterfaces()
        {
            /* Given */
            InterfaceDefinition original = @"interface Inf";

            /* When */
            var modified = original
                           .WithInterfaces(new List <NamedType>
            {
                "Inf1",
                "Inf2"
            });

            /* Then */
            Assert.Null(original.Interfaces);
            Assert.NotNull(modified.Interfaces);
            Assert.Equal(2, modified.Interfaces?.Count);
        }