# File lib/sprockets/manifest.rb, line 138
    def clean(keep = 2)
      self.assets.keys.each do |logical_path|
        # Get assets sorted by ctime, newest first
        assets = backups_for(logical_path)

        # Keep the last N backups
        assets = assets[keep..-1] || []

        # Remove old assets
        assets.each { |path, _| remove(path) }
      end
    end