# File lib/sass/plugin/staleness_checker.rb, line 39
      def initialize(options)
        @dependencies = self.class.dependencies_cache

        # URIs that are being actively checked for staleness. Protects against
        # import loops.
        @actively_checking = Set.new

        # Entries in the following instance-level caches are never explicitly expired.
        # Instead they are supposed to automaticaly go out of scope when a series of staleness checks
        # (this instance of StalenessChecker was created for) is finished.
        @mtimes, @dependencies_stale, @parse_trees = {}, {}, {}
        @options = Sass::Engine.normalize_options(options)
      end