check foreign constraints before delete

hi,

i want to write a function in plpgsql, which checks a foreign key constraint
violation on deleting a special value in my table. for example

CREATE TABLE to (n NAME PRIMARY KEY);
CREATE TABLE t1 (n_name NAME REFERENCES to(n), id INTEGER ...);

CREATE FUNCTION check_before_delete(..some value ...) RETURNS INTEGER AS '
..
IF foreign_key_violation THEN RETURN -1 END IF;
RETURN 0;
..
END;
...

so i can avoid the database errormessages.

i checked some 'd' commands with the 'E' option. but i have no idea for a
smart solution of this problem

thanx in advance (tom?)

sepp