diff --git a/src/moduleutils/PhutilLibraryMapBuilder.php b/src/moduleutils/PhutilLibraryMapBuilder.php
index af63ca3..735b172 100644
--- a/src/moduleutils/PhutilLibraryMapBuilder.php
+++ b/src/moduleutils/PhutilLibraryMapBuilder.php
@@ -1,511 +1,514 @@
 <?php
 
 /**
  * Build maps of libphutil libraries. libphutil uses the library map to locate
  * and load classes and functions in the library.
  *
  * @task map      Mapping libphutil Libraries
  * @task path     Path Management
  * @task symbol   Symbol Analysis and Caching
  * @task source   Source Management
  */
 final class PhutilLibraryMapBuilder extends Phobject {
 
   private $root;
   private $quiet = true;
   private $subprocessLimit = 8;
 
   private $fileSymbolMap;
   private $librarySymbolMap;
 
   const LIBRARY_MAP_VERSION_KEY   = '__library_version__';
   const LIBRARY_MAP_VERSION       = 2;
 
   const SYMBOL_CACHE_VERSION_KEY  = '__symbol_cache_version__';
   const SYMBOL_CACHE_VERSION      = 11;
 
 
 /* -(  Mapping libphutil Libraries  )---------------------------------------- */
 
   /**
    * Create a new map builder for a library.
    *
    * @param string Path to the library root.
    *
    * @task map
    */
   public function __construct($root) {
     $this->root = $root;
   }
 
   /**
    * Control status output. Use `--quiet` to set this.
    *
    * @param  bool  If true, don't show status output.
    * @return this
    *
    * @task map
    */
   public function setQuiet($quiet) {
     $this->quiet = $quiet;
     return $this;
   }
 
   /**
    * Control subprocess parallelism limit. Use `--limit` to set this.
    *
    * @param  int   Maximum number of subprocesses to run in parallel.
    * @return this
    *
    * @task map
    */
   public function setSubprocessLimit($limit) {
     $this->subprocessLimit = $limit;
     return $this;
   }
 
   /**
    * Get the map of symbols in this library, analyzing the library to build it
    * if necessary.
    *
    * @return map<string, wild> Information about symbols in this library.
    *
    * @task map
    */
   public function buildMap() {
     if ($this->librarySymbolMap === null) {
       $this->analyzeLibrary();
     }
     return $this->librarySymbolMap;
   }
 
 
   /**
    * Get the map of files in this library, analyzing the library to build it
    * if necessary.
    *
    * Returns a map of file paths to information about symbols used and defined
    * in the file.
    *
    * @return map<string, wild> Information about files in this library.
    *
    * @task map
    */
   public function buildFileSymbolMap() {
     if ($this->fileSymbolMap === null) {
       $this->analyzeLibrary();
     }
     return $this->fileSymbolMap;
   }
 
   /**
    * Build and update the library map.
    *
    * @return void
    *
    * @task map
    */
   public function buildAndWriteMap() {
     $library_map = $this->buildMap();
 
     $this->log(pht('Writing map...'));
     $this->writeLibraryMap($library_map);
   }
 
   /**
    * Write a status message to the user, if not running in quiet mode.
    *
    * @param  string  Message to write.
    * @return this
    *
    * @task map
    */
   private function log($message) {
     if (!$this->quiet) {
       @fwrite(STDERR, "%s\n", $message);
     }
     return $this;
   }
 
 
 /* -(  Path Management  )---------------------------------------------------- */
 
   /**
    * Get the path to some file in the library.
    *
    * @param  string  A library-relative path. If omitted, returns the library
    *                 root path.
    * @return string  An absolute path.
    *
    * @task path
    */
   private function getPath($path = '') {
     return $this->root.'/'.$path;
   }
 
   /**
    * Get the path to the symbol cache file.
    *
    * @return string Absolute path to symbol cache.
    *
    * @task path
    */
   private function getPathForSymbolCache() {
     return $this->getPath('.phutil_module_cache');
   }
 
   /**
    * Get the path to the map file.
    *
    * @return string Absolute path to the library map.
    *
    * @task path
    */
   private function getPathForLibraryMap() {
     return $this->getPath('__phutil_library_map__.php');
   }
 
   /**
    * Get the path to the library init file.
    *
    * @return string Absolute path to the library init file
    *
    * @task path
    */
   private function getPathForLibraryInit() {
     return $this->getPath('__phutil_library_init__.php');
   }
 
 
 /* -(  Symbol Analysis and Caching  )---------------------------------------- */
 
   /**
    * Load the library symbol cache, if it exists and is readable and valid.
    *
    * @return dict  Map of content hashes to cache of output from
    *               `phutil_symbols.php`.
    *
    * @task symbol
    */
   private function loadSymbolCache() {
     $cache_file = $this->getPathForSymbolCache();
 
     try {
       $cache = Filesystem::readFile($cache_file);
     } catch (Exception $ex) {
       $cache = null;
     }
 
     $symbol_cache = array();
     if ($cache) {
       try {
         $symbol_cache = phutil_json_decode($cache);
       } catch (PhutilJSONParserException $ex) {
         $symbol_cache = array();
       }
     }
 
     $version = idx($symbol_cache, self::SYMBOL_CACHE_VERSION_KEY);
     if ($version != self::SYMBOL_CACHE_VERSION) {
       // Throw away caches from a different version of the library.
       $symbol_cache = array();
     }
     unset($symbol_cache[self::SYMBOL_CACHE_VERSION_KEY]);
 
     return $symbol_cache;
   }
 
   /**
    * Write a symbol map to disk cache.
    *
    * @param  dict  Symbol map of relative paths to symbols.
    * @param  dict  Source map (like @{method:loadSourceFileMap}).
    * @return void
    *
    * @task symbol
    */
   private function writeSymbolCache(array $symbol_map, array $source_map) {
     $cache_file = $this->getPathForSymbolCache();
 
     $cache = array(
       self::SYMBOL_CACHE_VERSION_KEY => self::SYMBOL_CACHE_VERSION,
     );
 
     foreach ($symbol_map as $file => $symbols) {
       $cache[$source_map[$file]] = $symbols;
     }
 
     $json = json_encode($cache);
     try {
       Filesystem::writeFile($cache_file, $json);
     } catch (FilesystemException $ex) {
       $this->log(pht('Unable to save the cache!'));
     }
   }
 
   /**
    * Drop the symbol cache, forcing a clean rebuild.
    *
    * @return this
    *
    * @task symbol
    */
   public function dropSymbolCache() {
     $this->log(pht('Dropping symbol cache...'));
     Filesystem::remove($this->getPathForSymbolCache());
   }
 
   /**
    * Build a future which returns a `phutil_symbols.php` analysis of a source
    * file.
    *
    * @param  string  Relative path to the source file to analyze.
    * @return Future  Analysis future.
    *
    * @task symbol
    */
   private function buildSymbolAnalysisFuture($file) {
     $absolute_file = $this->getPath($file);
     $bin = dirname(__FILE__).'/../../scripts/phutil_symbols.php';
 
     return new ExecFuture('php %s --ugly -- %s', $bin, $absolute_file);
   }
 
 
 /* -(  Source Management  )-------------------------------------------------- */
 
   /**
    * Build a map of all source files in a library to hashes of their content.
    * Returns an array like this:
    *
    *   array(
    *     'src/parser/ExampleParser.php' => '60b725f10c9c85c70d97880dfe8191b3',
    *     // ...
    *   );
    *
    * @return dict  Map of library-relative paths to content hashes.
    * @task source
    */
   private function loadSourceFileMap() {
     $root = $this->getPath();
 
     $init = $this->getPathForLibraryInit();
     if (!Filesystem::pathExists($init)) {
       throw new Exception(
         pht(
           "Provided path '%s' is not a %s library.",
           $root,
           'phutil'));
     }
 
     $files = id(new FileFinder($root))
       ->withType('f')
       ->withSuffix('php')
       ->excludePath('*/.*')
       ->setGenerateChecksums(true)
       ->find();
 
+    $extensions_dir = 'extensions/';
+    $extensions_len = strlen($extensions_dir);
+
     $map = array();
     foreach ($files as $file => $hash) {
       $file = Filesystem::readablePath($file, $root);
       $file = ltrim($file, '/');
 
       if (dirname($file) == '.') {
         // We don't permit normal source files at the root level, so just ignore
         // them; they're special library files.
         continue;
       }
 
-      if (dirname($file) == 'extensions') {
-        // Ignore files in the extensions/ directory.
+      // Ignore files in the extensions/ directory.
+      if (!strncmp($file, $extensions_dir, $extensions_len)) {
         continue;
       }
 
       // We include also filename in the hash to handle cases when the file is
       // moved without modifying its content.
       $map[$file] = md5($hash.$file);
     }
 
     return $map;
   }
 
   /**
    * Convert the symbol analysis of all the source files in the library into
    * a library map.
    *
    * @param   dict  Symbol analysis of all source files.
    * @return  dict  Library map.
    * @task source
    */
   private function buildLibraryMap(array $symbol_map) {
     $library_map = array(
       'class'     => array(),
       'function'  => array(),
       'xmap'      => array(),
     );
 
     // Detect duplicate symbols within the library.
     foreach ($symbol_map as $file => $info) {
       foreach ($info['have'] as $type => $symbols) {
         foreach ($symbols as $symbol => $declaration) {
           $lib_type = ($type == 'interface') ? 'class' : $type;
           if (!empty($library_map[$lib_type][$symbol])) {
             $prior = $library_map[$lib_type][$symbol];
             throw new Exception(
               pht(
                 "Definition of %s '%s' in file '%s' duplicates prior ".
                 "definition in file '%s'. You can not declare the ".
                 "same symbol twice.",
                 $type,
                 $symbol,
                 $file,
                 $prior));
           }
           $library_map[$lib_type][$symbol] = $file;
         }
       }
       $library_map['xmap'] += $info['xmap'];
     }
 
     // Simplify the common case (one parent) to make the file a little easier
     // to deal with.
     foreach ($library_map['xmap'] as $class => $extends) {
       if (count($extends) == 1) {
         $library_map['xmap'][$class] = reset($extends);
       }
     }
 
     // Sort the map so it is relatively stable across changes.
     foreach ($library_map as $key => $symbols) {
       ksort($symbols);
       $library_map[$key] = $symbols;
     }
     ksort($library_map);
 
     return $library_map;
   }
 
   /**
    * Write a finalized library map.
    *
    * @param  dict Library map structure to write.
    * @return void
    *
    * @task source
    */
   private function writeLibraryMap(array $library_map) {
     $map_file = $this->getPathForLibraryMap();
     $version = self::LIBRARY_MAP_VERSION;
 
     $library_map = array(
       self::LIBRARY_MAP_VERSION_KEY => $version,
     ) + $library_map;
 
     $library_map = phutil_var_export($library_map);
     $at = '@';
 
     $source_file = <<<EOPHP
 <?php
 
 /**
  * This file is automatically generated. Use 'arc liberate' to rebuild it.
  *
  * {$at}generated
  * {$at}phutil-library-version {$version}
  */
 phutil_register_library_map({$library_map});
 
 EOPHP;
 
     Filesystem::writeFile($map_file, $source_file);
   }
 
 
   /**
    * Analyze the library, generating the file and symbol maps.
    *
    * @return void
    */
   private function analyzeLibrary() {
     // Identify all the ".php" source files in the library.
     $this->log(pht('Finding source files...'));
     $source_map = $this->loadSourceFileMap();
     $this->log(
       pht('Found %s files.', new PhutilNumber(count($source_map))));
 
     // Load the symbol cache with existing parsed symbols. This allows us
     // to remap libraries quickly by analyzing only changed files.
     $this->log(pht('Loading symbol cache...'));
     $symbol_cache = $this->loadSymbolCache();
 
     // If the XHPAST binary is not up-to-date, build it now. Otherwise,
     // `phutil_symbols.php` will attempt to build the binary and will fail
     // miserably because it will be trying to build the same file multiple
     // times in parallel.
     if (!PhutilXHPASTBinary::isAvailable()) {
       PhutilXHPASTBinary::build();
     }
 
     // Build out the symbol analysis for all the files in the library. For
     // each file, check if it's in cache. If we miss in the cache, do a fresh
     // analysis.
     $symbol_map = array();
     $futures = array();
     foreach ($source_map as $file => $hash) {
       if (!empty($symbol_cache[$hash])) {
         $symbol_map[$file] = $symbol_cache[$hash];
         continue;
       }
       $futures[$file] = $this->buildSymbolAnalysisFuture($file);
     }
     $this->log(
       pht('Found %s files in cache.', new PhutilNumber(count($symbol_map))));
 
     // Run the analyzer on any files which need analysis.
     if ($futures) {
       $limit = $this->subprocessLimit;
 
       $this->log(
         pht(
           'Analyzing %s file(s) with %s subprocess(es)...',
           phutil_count($futures),
           new PhutilNumber($limit)));
 
       $progress = new PhutilConsoleProgressBar();
       if ($this->quiet) {
         $progress->setQuiet(true);
       }
       $progress->setTotal(count($futures));
 
       $futures = id(new FutureIterator($futures))
         ->limit($limit);
       foreach ($futures as $file => $future) {
         $result = $future->resolveJSON();
         if (empty($result['error'])) {
           $symbol_map[$file] = $result;
         } else {
           $progress->done(false);
           throw new XHPASTSyntaxErrorException(
             $result['line'],
             $file.': '.$result['error']);
         }
         $progress->update(1);
       }
       $progress->done();
     }
 
     $this->fileSymbolMap = $symbol_map;
 
     // We're done building the cache, so write it out immediately. Note that
     // we've only retained entries for files we found, so this implicitly cleans
     // out old cache entries.
     $this->writeSymbolCache($symbol_map, $source_map);
 
     // Our map is up to date, so either show it on stdout or write it to disk.
     $this->log(pht('Building library map...'));
 
     $this->librarySymbolMap = $this->buildLibraryMap($symbol_map);
   }
 
 
 }