RosettaCodeData/Task/Fibonacci-sequence/SequenceL/fibonacci-sequence-2.sequencel

fibonacci(n) := fibonacciHelper(0, 1, n);

fibonacciHelper(prev, next, n) :=
      prev when n < 1
   else
      next when n = 1
   else
      fibonacciHelper(next, next + prev, n - 1);