Problem about Tuple.opEquals, const qualifier

[Tongzhou Li]

I'm learning D, and trying to convert My C++ code into D:
http://pastebin.com/eCz9DdZ3
I wrote: auto stack = SList!(Tuple!(double, char))();
But I got an error
Error: function 
std.typecons.Tuple!(double,char).Tuple.opEquals!(const(Tuple!(double,char))).opEquals 
(const(Tuple!(double,char)) rhs) is not callable using argument 
types (const(Tuple!(double,char))) const
Does the function Tuple.opEquals miss a const qualifier?
Sorry for my bad English:)