cascading backwards

If a table A contains a foreign key to table B is it possible to have
a DELETE remove the targetted row in B when there are no longer any
other references in A to it? Either with SQL or using some other
PostgreSQL facility.

As an example,

Body:
body_id data
1       "some news"

Page:
page_id body_id name
10      1       "news"
11      1       "recent news"

If page_id=10 is deleted, nothing extra happens. If page_id=11 is
deleted, body_id=1 goes with it.

Many thanks, Paul

--
Paul Makepeace ....................................... http://paulm.com/

"If kangaroos graduate, then it shall rain in ghana on my sweet
 potatoes."
   -- http://paulm.com/toys/surrealism/