code: def add_variables_from_directory(self, directory):
          
          py_files = glob.glob(path.join(directory, "*.py"))
          for py_file in py_files:
              self.add_variables_from_file(py_file)
          subdirectories = glob.glob(path.join(directory, "*/"))
          for subdirectory in subdirectories:
              self.add_variables_from_directory(subdirectory)
docs: Recursively explores a directory, and adds all OpenFisca variables found there to the tax and benefit system.