filter_by

# File railties/lib/rails/paths.rb, line 101
      def filter_by(constraint)
        all = []
        all_paths.each do |path|
          if path.send(constraint)
            paths  = path.existent
            paths -= path.children.map { |p| p.send(constraint) ? [] : p.existent }.flatten
            all.concat(paths)
          end
        end
        all.uniq!
        all
      end