question on rules

hi folks,

i'd like to write an rule which fills out some empty attrs on 
insert (w/ data from other given attrs).

the table structure is:
CREATE TABLE foo (start date, duration integer, endtime date);

and the rule is:
CREATE RULE foo1 AS ON INSERT TO foo WHERE new.endtime = NULL DO    INSERT INTO foo SELECT new.start, new.duration,
new.start+ new.duration;   
 
when i try to insert some data (INSERT INTO foo SELECT '2003-01-01', '12';)
i get the error: query rewritten 10 times, may contain cycles.

how can i avoid this loop ?

cu
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