code: def _environment_sanity_check(environment):
      
      assert issubclass(environment.undefined, Undefined), 'undefined must ' \
          'be a subclass of undefined because filters depend on it.'
      assert environment.block_start_string != \
          environment.variable_start_string != \
          environment.comment_start_string, 'block, variable and comment ' \
          'start strings must be different'
      assert environment.newline_sequence in ('\r', '\r\n', '\n'), \
          'newline_sequence set to unknown line ending string.'
      return environment
docs: Perform a sanity check on the environment.