Android :: How To Implement Custom AlertDialog View?

In the Android docs on AlertDialog, it gives the following instruction and example for setting a custom view in an AlertDialog:If you want to display a more complex view, look up the FrameLayout called "body" and add your view to it:First off, it's pretty obvious that add() is a typo and is meant to be addView().I'm confused by the first line using R.id.body. It seems that it's the body element of the AlertDialog but I can't just enter that in my code b/c it gives a compile error. Where does R.id.body get defined or assigned or whatever?