RedBlackTree and myClass

[AntonSotov via Digitalmars-d-learn]

import std.container.rbtree;

class myClass {
     string str;
}


int main()
{
     auto tree = new RedBlackTree!myClass;
     return 0;
}


Error: mutable method object.Object.opCmp is not callable using a 
inout object
Error: template instance std.functional.binaryFun!("a < b", "a", 
"b").binaryFun!(inout(myClass), myClass) error instantiating
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How to use RedBlackTree of the objects of of its class?