diff --git a/lib/Database.php b/lib/Database.php
index 2486907..df29fb4 100644
--- a/lib/Database.php
+++ b/lib/Database.php
@@ -700,7 +700,7 @@ class Database {
      * @param string $url The URL of the newsfeed or discovery source
      * @param string $fetchUser The user name required to access the newsfeed, if applicable
      * @param string $fetchPassword The password required to fetch the newsfeed, if applicable; this will be stored in cleartext
-     * @param boolean $discovery Whether to perform newsfeed discovery if $url points to an HTML document
+     * @param boolean $discover Whether to perform newsfeed discovery if $url points to an HTML document
      */
     public function subscriptionAdd(string $user, string $url, string $fetchUser = "", string $fetchPassword = "", bool $discover = true): int {
         if (!Arsse::$user->authorize($user, __FUNCTION__)) {
diff --git a/lib/ImportExport/OPML.php b/lib/ImportExport/OPML.php
index 06ebfb3..5a1da74 100644
--- a/lib/ImportExport/OPML.php
+++ b/lib/ImportExport/OPML.php
@@ -11,7 +11,8 @@ use JKingWeb\Arsse\User\Exception as UserException;
 
 class OPML {
     public function import(string $user, string $opml, bool $flat = false, bool $replace = false): bool {
-        list($folders, $feeds) = $this->parse($opml, $flat);
+        list($feeds, $folders) = $this->parse($opml, $flat);
+        
         return true;
     }
 
@@ -29,34 +30,47 @@ class OPML {
         $body = $body->item(0);
         $folders = [];
         $feeds = [];
+        // add the root folder to a map from folder DOM nodes to folder ID numbers
         $folderMap = new \SplObjectStorage;
         $folderMap[$body] = sizeof($folderMap);
+        // iterate through each node in the body
         $node = $body->firstChild;
         while ($node && $node != $body) {
             if ($node->nodeType == \XML_ELEMENT_NODE && $node->nodeName === "outline") {
+                // process any nodes which are outlines
                 if ($node->getAttribute("type") === "rss") {
+                    // feed nodes
                     $url = $node->getAttribute("xmlUrl");
                     if (strlen($url)) {
+                        // only process the node if it has a URL
                         $title = $node->getAttribute("text");
                         $folder = $folderMap[$node->parentNode] ?? 0;
                         $categories = $node->getAttribute("category");
                         if (strlen($categories)) {
+                            // collapse and trim whitespace from category names, if any, splitting along commas
                             $categories = array_map(function($v) {
                                 return trim(preg_replace("/\s+/g", " ", $v));
                             }, explode(",", $categories));
+                        } else {
+                            $categories = [];
                         }
                         $feeds[] = ['url' => $url, 'title' => $title, 'folder' => $folder, 'categories' => $categories];
                     }
+                    // skip any child nodes of a feed outline-entry
                     $node = $node->nextSibling ?: $node->parentNode;
                 } else {
+                    // any outline entries which are not feeds are treated as folders
                     if (!$flat) {
+                        // only process folders if we're not treating he file as flat
                         $id = sizeof($folderMap);
                         $folderMap[$node] = $id;
                         $folders[$id] = ['id' => $id, 'name' => $node->getAttribute("text"), 'parent' => $folderMap[$node->parentNode]];
                     }
+                    // proceed to child nodes, if any
                     $node = $node->hasChildNodes() ? $node->firstChild : ($node->nextSibling ?: $node->parentNode);
                 }
             } else {
+                // skip any node which is not an outline element; if the node has descendents they are skipped as well
                 $node = $node->nextSibling ?: $node->parentNode;
             }
         }