code: def existing_config_files():
      
      global _ETC_PATHS
      global _MAIN_CONFIG_FILE
      global _CONFIG_VAR_INCLUDE
      global _CONFIG_FILTER
  
      config_files = []
      for possible in _ETC_PATHS:
          config_files = config_files + glob.glob("%s%s" % (possible, _MAIN_CONFIG_FILE))
      
      if _CONFIG_VAR_INCLUDE != "":
          main_config = Configuration("general", {
              _CONFIG_VAR_INCLUDE:""
          }, _MAIN_CONFIG_FILE)
  
          if main_config.CONFIG_DIR != "":
              for possible in _ETC_PATHS:
                  config_files = config_files + glob.glob("%s%s/%s" % (possible, main_config.CONFIG_DIR, _CONFIG_FILTER))
              
      return config_files
docs: Method that calculates all the configuration files that are valid, according to the
          'set_paths' and other methods for this module.