How to create formulas in an SQL statement

michaels>  WITH t AS
     (SELECT 1 ID, 1 x, 2 y FROM DUAL UNION ALL
      SELECT 2,    3,   4   FROM DUAL UNION ALL
      SELECT 3,    5,   6   FROM DUAL)
-----      
SELECT   *
    FROM XMLTable('declare function local:a($a,$b)
                   {
                     ($a + $b)
                   }; (: eof :)
                   for $i in /ROWSET/ROW
                   return <ROW>
                          <ID>{$i/ID}</ID>
                          <X>{$i/X}</X>
                          <Y>{$i/Y}</Y>
                          <A>{local:a($i/X,$i/Y)}</A>
                          <B>{local:a($i/X,$i/Y) + 2}</B>
                          </ROW>' PASSING XMLTYPE(CURSOR(SELECT * FROM t))
                   COLUMNS ID NUMBER PATH 'ID',
                            X NUMBER PATH 'X',
                            Y NUMBER PATH 'Y',
                            A NUMBER PATH 'A',
                            B NUMBER PATH 'B'
                   )

        ID          X          Y          A          B
---------- ---------- ---------- ---------- ----------
         1          1          2          3          5
         2          3          4          7          9
         3          5          6         11         13