If else state on Hiding a button in swift

I have two basic buttons on my storyboard in swift. If you hit the button on the left the button on the right will always show or hide. I have defined outlets for both buttons but my if else statement is wrong. This is my if else statement.

@IBAction func aa(sender: AnyObject) {

    if {
        b.hidden = true
    } else {
        b.hidden = false
    }

}