Operator Overloading in an Abstract Base Class

["Nordlöw" via Digitalmars-d-learn]

In my module symbolic.d at

https://github.com/nordlow/justd/blob/master/symbolic.d

is it somehow possible to overload binary operator ~ on pairs of 
instances SPatt?

I'm asking because my current try at

https://github.com/nordlow/justd/blob/master/symbolic.d#L332

is not called in the unittest at

https://github.com/nordlow/justd/blob/master/symbolic.d#L347

Isn't it possible to do operator overloading in an abstract 
base-class?