DLang Tour : Functions as arguments

[Tony via Digitalmars-d-learn]

On this page:
https://tour.dlang.org/tour/en/basics/delegates

there is:
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void doSomething(int function(int, int) doer) {
    // call passed function
    doer(5,5);
}

doSomething(add); // use global function `add` here
                  // add must have 2 int parameters

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I can't get it to compile unless it is:

doSomething(&add);