A$={{ Module "Fibonacci" : Read X  :If X<0 then {Error {X<0}} Else  Fib=Lambda (x)->if(x>1->fib(x-1)+fib(x-2), x) : =fib(x)}}
Try Ok {
      Print Function(A$, -12)
}
If Error or Not Ok Then Print Error$
Print Function(A$, 12)=144 ' true