def add_entry(e)
      @entries.push(e)
      if @md5_to_entries.has_key?(e.md5)
        @md5_to_entries[e.md5].push(e)
      else
        @md5_to_entries[e.md5] = [e]
      end
    end
def add_entry(e)
      @entries.push(e)
      if @md5_to_entries.has_key?(e.md5)
        @md5_to_entries[e.md5].push(e)
      else
        @md5_to_entries[e.md5] = [e]
      end
    end
add entry to this catalog
 @private