[Vala] How to connect a delegate to a signal?

["G.S.Alex" <sthots gmail com>]

I used to think the delegate could be used just like method it self .
But when I try to connect a delegate to a signal , i got the error:

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t.vala:24.29-24.29: error: Argument 1: Cannot convert from `DN' to
`Foo.some_event'
    foo.some_event.connect (d);
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So, how could I do that?

my test code:
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public class Foo : Object {
    public signal void some_event ();   // definition of the signal

    public void method () {
        some_event ();                  // emitting the signal
(callbacks get invoked)
    }
}

delegate void DN();

static void callback_a () {
    stdout.printf ("Callback A\n");
}

static void callback_b () {
    stdout.printf ("Callback BB\n");
}

static int main (string[] args) {
    var foo = new Foo ();
    foo.some_event.connect (callback_a);      // connecting the
callback functions
    DN d = callback_b;
    //d();
    foo.some_event.connect (d);
    foo.method ();
    return 0;
}
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