callbacks = new Callbacks("memory once unique");
callbacks.add( fn1 );
callbacks.add( fn1 );
callbacks.fire( "bar" );
assertEquals(" f1: bar", result);
callbacks.fire( "foo" );
assertEquals(" f1: bar", result);
callbacks.add( fn2 );
callbacks.add( fn2 );
assertEquals(" f1: bar f2: bar f2: bar", result);
callbacks.remove( fn1 );