Photoshop :: Reversing A Selection?

Sometimes this happens to me in using selection tools, especially the Lasso tools where I want to subtract from or add toan existing selection, but I hit the wrong key and end up adding to it instead.
 
Then, you undo, make your selection again and hopefully you've rememberd to hit the correct modifyer key..
 
Is there a way to, after making your selection, to tell photoshop to do the opposite of what you've just told it to? In other words, if you added when you meant to subtract, have PS do the opposite function, but using the same selection you made, so that you don't have to do that part again?