lex_match (lexer, T_BY);
if (!parse_variables (lexer, dataset_dict (ds), &v, &n, PV_NO_DUPLICATE))
return CMD_CASCADING_FAILURE;
lex_match (lexer, T_BY);
if (!parse_variables (lexer, dataset_dict (ds), &v, &n, PV_NO_DUPLICATE))
return CMD_CASCADING_FAILURE;