fib n: Int =
    if n <= 1, do:
        fib.return n
    fib(n - 1) + fib(n - 2)

assert fib(10) == 55

# Need to specify the return type
invalid_fib(n: Int): Int or Str =
    if n <= 1, do:
        invalid_fib.return n
    "string"