templates nthFibonacci
  @: {n0: 0"1", n1: 1"1"};
  def sign: $ -> \(<0..> 1! <> -1!\);
  1..$*$sign -> $sign -> #
  $@.n0!
  <=1>
    @: {n0: $@.n1, n1: $@.n0 + $@.n1};
  <=-1>
    @: {n0: $@.n1 - $@.n0, n1: $@.n0};
end nthFibonacci