# File lib/rgen/util/file_cache_map.rb, line 80
  def clean_unused(root_path, key_paths)
    raise "key paths must be within root path" unless key_paths.all?{|p| p.index(root_path) == 0}
    used_files = key_paths.collect{|p| cache_file(p)}
    files = Dir[root_path+"/**/"+@cache_dir+"/*"+@postfix] 
    files.each do |f|
      FileUtils.rm(f) unless used_files.include?(f)
    end
  end