Function fib(x) {
      If x<0 then Error "argument outside of range"
      If x<2 then =x : exit
      Def fib1(x)=If(x>1->lambda(x-1)+lambda(x-2), x)
      =fib1(x)
}
Module CheckIt (&k()) {
      Print k(12)
}
CheckIt &Fib()
Print fib(-2)  ' error