Closing Gaps in Celko trees

Hi All

Im trying to convert one of the functions the Celko wrote for closing gaps
in lft,rgt tree type.

The function return me: ERROR: parser: parse error at or near "while"

Any ideas ?

# This is Celko sample from book

BEGIN
WHILE EXISTS ( SELECT * FROM gaps )
LOOP UPDATE frammis
SET rgt = CASE WHEN rgt > ( SELECT MIN(start) FROM gaps )
THEN rgt - 1 ELSE rgt END,
lft = CASE WHEN lft > ( SELECT MIN(start) FROM gaps )
THEN lft - 1 ELSE lft END;
END WHILE;

# This is me trying to covert it to PG

CREATE FUNCTION close_gaps() RETURNS TEXT AS '
DECLARE
right_most_sibling INTEGER;

BEGIN
WHILE EXISTS ( SELECT * FROM gaps )
LOOP UPDATE tree
SET rgt = CASE
WHEN rgt > ( SELECT MIN(start) FROM gaps )
THEN rgt - 1 ELSE rgt END,
lft = CASE
WHEN lft > ( SELECT MIN(start) FROM gaps )
THEN lft - 1 ELSE lft END;
END WHILE;

RETURN NEW;

END;'
LANGUAGE 'plpgsql';