How to handle recursion in XQuery?

Question

I am trying to find all countries that are reachable by land, moving from one country to another across land borders using the mondial.sql database. This needs to be done recursively, and I found some features on the Internet that I thought would be useful for joining sequences and in order to exclude countries that were already found.

The problem is that I am ending the cycle, although the countries that should be excluded seem to be handled properly. Therefore, I think that I may have to somehow determine the base case in order to stop the recursion as soon as all possible countries are found. How to achieve this with XQuery?

(:functx.value-union and is-value-in-sequence were found at http://www.xqueryfunctions.com/xq/:)
declare namespace functx = "http://www.functx.com";
declare function functx:value-union
  ( $arg1 as xs:anyAtomicType* ,
    $arg2 as xs:anyAtomicType* )  as xs:anyAtomicType* {

  distinct-values(($arg1, $arg2))
 };

 declare function functx:is-value-in-sequence
  ( $value as xs:anyAtomicType? ,
    $seq as xs:anyAtomicType* )  as xs:boolean {

   $value = $seq
 } ;

(:Recursive function for finding reachable countries:)
declare function local:findReachable($countries, $country, $reachedCountries) {
    let $reachableCountries := $countries[@car_code = $country/border/@country]
    for $c in $reachableCountries
    where not(functx:is-value-in-sequence($c, $reachedCountries))
    return functx:value-union($c, local:findReachable($countries, $c, functx:value-union($reachableCountries, 
    $reachedCountries)))
};

let $countries := //country
let $startingCountry := //country[@car_code = 'S']
return local:findReachable($countries, $startingCountry, $startingCountry)

Answer 1

$reachedCountries , , - , . , .

, , :

declare function local:dfs($stack, $seen) {
  if(empty($stack)) then $seen
  else (
    let $country := $stack[1]
    let $neighbors :=
        for $code in $country/border/@country[not(. = $seen/@car_code)]
        return $country/../country[@car_code = $code]
    return local:dfs(($neighbors, $stack[position() > 1]), ($seen, $neighbors))
  )
};

local:dfs(doc('mondial.xml')//country[@car_code = 'S'], ())/name