nice sql problem

given table contains
select * from z;

         A B
---------- ----------

         1          2
         2          3
         3          4
         7          8
         8         10
        12         13

wanted output should contain
         A          B


---------- ----------

         1          4
         7         10
        12         13

how ? in plain sql - no plsql.

select min(a) a, max(b) b
from z
connect by prior b = a
start with a = 1
/

         A B
---------- ----------

         1 4

is not complete. Any suggestions ?

tnx,
Ronald.