:thread return -x Error Encountered a crash

Im new to Xcode getting this error in this playground, can someone help me here... This was In a playground btw Thanks,

struct Digit {
    var number : Int
    init(_ n: Int){
        self.number = n
    }
    mutating func changeNumberTo(_ n:Int) {
        self.number = n
    }
    func otherFunction(_ f: ()->()) {
    }
    mutating func callAnotherFunction() {
        otherFunction {
            self.changeNumberTo(345) // *
             }
    }
}










var d = Digit(123) // Digit is a struct
print(d.number) //123
var d2 = d //Assignment!
d2.number = 42
print(d.number)





enum Node {
    case none(Int)
    indirect case left(Int, Node)
    indirect case right(Int, Node)
    indirect case both(Int, Node, Node)
}