How can I save a class use the RedBlackTree?

[Dsby via Digitalmars-d-learn]

this is the test Code:

import std.container.rbtree;
import std.stdio;

class TClass
{
    int i;
}

void main()
{
    RedBlackTree!(TClass) list = new RedBlackTree!(TClass)();
    auto t = new TClass();
    list.insert(t);
    writeln("The rbtree length is ",list.length());
    list.removeKey(t);
    writeln("The rbtree length is ",list.length());
}

and thisis erro :
/usr/include/dlang/dmd-2.071.0/phobos/std/functional.d-mixin-206(206): Error: 
mutable method object.Object.opCmp is not callable using a inout object
/usr/include/dlang/dmd-2.071.0/phobos/std/container/rbtree.d(871): Error: template instance 
std.functional.binaryFun!("a < b", "a", "b").binaryFun!(inout(TClass), TClass) 
error instantiating
rbtree.d(11):        instantiated from here: 
RedBlackTree!(TClass, "a < b", false)
/usr/include/dlang/dmd-2.071.0/phobos/std/functional.d-mixin-206(206): Error: 
function object.Object.opCmp (Object o) is not callable using argument types 
(inout(TClass))
/usr/include/dlang/dmd-2.071.0/phobos/std/container/rbtree.d(873): Error: template instance 
std.functional.binaryFun!("a < b", "a", "b").binaryFun!(TClass, inout(TClass)) 
error instantiating
rbtree.d(11):        instantiated from here: 
RedBlackTree!(TClass, "a < b", false)