docs: def _varslist2axis(cls, fluent: 'TensorFluent', vars_list: List[str]) -> List[int]:
          '''Maps the `vars_list` into a list of axis indices
          corresponding to the `fluent` scope.
  
          Args:
              x: The fluent.
              vars_list: The list of variables to be aggregated over.
  
          Returns:
              List[int]: a list of axis.
          '''
          axis = []
          for var in vars_list:
              if var in fluent.scope.as_list():
                  ax = fluent.scope.index(var)
                  if fluent.batch:
                      ax += 1
                  axis.append(ax)
          return axis