problem with overloading the "coalesce" function

Hi,
I am trying to overload the "coalesce" function to
accept an integer and a string. Here it is :

CREATE OR REPLACE FUNCTION "coalesce"(a int4, b
"varchar")
RETURNS "varchar" AS
$BODY$
begin
if (a is null ) then
return b;
else
return cast(a as varchar(15));
end if;
end
$BODY$
LANGUAGE 'plpgsql' VOLATILE;
ALTER FUNCTION "coalesce"(a int4, b "varchar") OWNER
TO postgres;

I have added it to pg_catalog, but still I cant't use
it, I get an error on the second parameter, apparently
the function gets lost at some point. Any additional
steps I need to complete?

Thanks in advance,
Emil

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