Why is it an error to have a singly-linked list of Tuple!(int)? phobos\std\container.d(938): Error: function std.typecons.Tuple!(int).Tuple .opEquals!(const(Tuple!(int))) .opEquals (const(Tuple!(int)) rhs) is not callable using argument types (const(Tuple!(int))) const Error: template instance std.container.SList!(Tuple!(int)) error instantiating