code: def filter_(input_, filename='<internal>', state='INITIAL'):
      
      global CURRENT_DIR
  
      prev_dir = CURRENT_DIR
      CURRENT_FILE.append(filename)
      CURRENT_DIR = os.path.dirname(CURRENT_FILE[-1])
      LEXER.input(input_, filename)
      LEXER.lex.begin(state)
      parser.parse(lexer=LEXER, debug=OPTIONS.Debug.value > 2)
      CURRENT_FILE.pop()
      CURRENT_DIR = prev_dir
docs: Filter the input string thought the preprocessor.
      result is appended to OUTPUT global str