code: def add_io_macro(self,io,filename):
      
      io = self.__bad_macro_chars.sub( r'', io )
      if io not in self.__opts:
        self.__opts[io] = filename
      else:
        if filename not in self.__opts[io]:
          self.__opts[io] += ',%s' % filename
docs: Add a variable (macro) for storing the input/output files associated
      with this node.
      @param io: macroinput or macrooutput
      @param filename: filename of input/output file