How to use the fxjs.flatten function in fxjs

function(groups) {
                  each(function(result) {
                    result._ = result._ || {};
                    result._[me.as] = (groups[result[me.left_key]] || []);
                  }, results);
                  return recur([me, cat(map(r => r._ ? r._[me.as] : null, results))]);
                }
              );

(query) => left.row_number.length == 2  ?
        QUERY `
        SELECT "--row_number_table--".*
        FROM (
          SELECT 
            ${COLUMN(...cat(join_columns))},
            ROW_NUMBER() OVER (PARTITION BY ${CL(left.as + '.' + left.key)} ORDER BY ${left.row_number[1]}) as "--row_number--"
          FROM ${TB(left.table)} AS ${TB(left.as)}
          ${SQLS(join_sqls)}
          ${where_in || tag()}
          ${tag(query)}
        ) AS "--row_number_table--"
        WHERE "--row_number_table--"."--row_number--"<=${left.row_number[0]}`
        :
        QUERY `
          SELECT ${COLUMN(...cat(join_columns))}
          FROM ${TB(left.table)} AS ${TB(left.as)}
          ${SQLS(join_sqls)}
          ${where_in || tag()}
          ${tag(query)}`,
      left.row_number.length == 2 ? map(r => delete r['--row_number--'] && r) : r => r,