[TYPO3] How does ifEmpty work? Simple TypoScript question

[Benjamin Mack]

Hey everybody,

I just wondered about the "if" Functions. I am able to make nothing of 
it. The documentation (in regard of the posts about that the last days) 
does not help me really about that.

What I want is, that if my MENU object is empty, that it overrides the 
sidemenu.wrap function. That's why I got this "wrap" on line 3, and it 
should be overridden in the last line. Could someone please help me on this?
----
lib.sidemenu = HMENU
lib.sidemenu.entryLevel = 2
lib.sidemenu.wrap = <div id="sidenav"> | </div>
lib.sidemenu.1 = TMENU
lib.sidemenu.1 {
  noBlur = 1
  NO = 1
  ACT = 1
}
lib.sidemenu.stdWrap.ifEmpty.wrap = |
----

Thanks a bunch!

-- 
greetings,
benni.
-SDG-