proc fcnF(n : int) : tuple[a, b :int] =
  if n == 0: (1,2)
  else:
    let x = fcnF(n-1)
    (x.b, 6 * x.a * x.b div (5 * x.a - x.b))

echo fcnF(30) # ok (a: 1073741824, b: 2147483648)
echo fcnF(31) # Error: unhandled exception: over- or underflow [OverflowError]