-- helper function
fib' a b accum i x =
  if x == 0 then 1
  else if x == 1 then 1
  else if i == x then accum
  else (fib' b (a + b) (a + b) (i + 1) x)

-- calculate the xth fibonacci number
fib x =
  fib' 0 1 0 0 x