XQuery tail recursive Fibonacci function

fib.xq

declare function local:fib($n as xs:integer, $a as xs:integer, $b as xs:integer){ 
    switch ($n)
    case 0 return $a
    case 1 return $b
   default return local:fib($n - 1, $b, $a + $b)
};

declare function local:fib($n as xs:integer){ 
  local:fib($n,0,1)
};