fun fibonacci = int by int n
  if n < 0 do
    logLine("Invalid argument: " + n) # logs on standard error
	return -1 ^| it should be better to raise an error,
	           | but the task is about recursive functions
	           |^
  end
  fun actualFibonacci = int by int n
    return when(n < 2, n, actualFibonacci(n - 1) + actualFibonacci(n - 2))
  end
  return actualFibonacci(n)
end
writeLine("F(0)   = " + fibonacci(0))
writeLine("F(20)  = " + fibonacci(20))
writeLine("F(-10) = " + fibonacci(-10))
writeLine("F(30)  = " + fibonacci(30))
writeLine("F(10)  = " + fibonacci(10))