[Moo] How to write it (Syntax-Problem)

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Hello again,
one more problem.
How I have to write the sp variable in the right way to get the $
(div) ?

var panel = $$('.panel');

var sp = "nav-"+ panel.get('id');  // gives nav-hello

$(+'sp'+).setClass('active');      //wrong!!!!!!!!!  shoud be $('nav-
hello').setClass('active');