How use Predicate (alias pred = "a*b")?

[Marcone via Digitalmars-d-learn]

import std;

template foo(alias pred = "a*b"){
        void foo(int x, int y){
                writeln(x.unaryFun!pred);
        }
}

void main(){
        foo(5, 4);
}


"a" works, but "b" not work.
I get this error: Error: undefined identifier `b`