fn fib1(n, a, b) {
  if n == 0 { a+b }
  else { fib1(n-1, a+b, a) }
}

fn fib(n) {
  if n <= 0 { 0 }
  else {
    if n <= 2 { 1 }
    else {
      fib1(n-2, 1, 0)
    }
  }
}

fib(42) == 267914296
&& fib(84) == 160500643816367088
// overflows stack in debug mode, works in release mode
//&& fib(92) == 7540113804746346429
// fib(93) overflows i64