fun fibonacci ← int by int n
  if n < 0
    logLine("Invalid argument: " + n)
    return -1 # we could raise an error instead
  end
  fun actualFibonacci ← <int n|when(n < 2,
    n, actualFibonacci(n - 1) + actualFibonacci(n - 2))
  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))