code: def cat_core(list_of_columns, sep):
      
      list_with_sep = [sep] * (2 * len(list_of_columns) - 1)
      list_with_sep[::2] = list_of_columns
      return np.sum(list_with_sep, axis=0)
docs: Auxiliary function for :meth:`str.cat`
  
      Parameters
      ----------
      list_of_columns : list of numpy arrays
          List of arrays to be concatenated with sep;
          these arrays may not contain NaNs!
      sep : string
          The separator string for concatenating the columns
  
      Returns
      -------
      nd.array
          The concatenation of list_of_columns with sep