def cat_core(list_of_columns, sep):
list_with_sep = [sep] * (<NUM_LIT:2> * len(list_of_columns) - <NUM_LIT:1>)<EOL>list_with_sep[::<NUM_LIT:2>] = list_of_columns<EOL>return np.sum(list_with_sep, axis=<NUM_LIT:0>)<EOL>
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