User-defined function with anyrange[]

Hello,

I want to make an aggregate function range_agg(anyrange) that returns
anyrange[]. But when I try to define it, Postgres tells me it doesn't
know what an anyrange[] is. I get this error:

ERROR: type anyrange[] does not exist

I also tried taking an anyrange and returning an anyarray, which does
let me define the function, but upon calling it I get an error. For
example:

paul=# CREATE OR REPLACE FUNCTION range_agg4(anyrange)
RETURNS anyarray
AS $$
BEGIN
RETURN ARRAY[$1];
END;
$$
LANGUAGE plpgsql;

paul=# select range_agg4(daterange('2016-05-04', '2016-07-01'));
ERROR: invalid input syntax for type date: "[2016-05-04,2016-07-01)"
CONTEXT: PL/pgSQL function range_agg4(anyrange) while casting
return value to function's return type

So I guess it thinks that daterange in means date[] out (not daterange[] out).

The best I can think of is to define range_agg(anyelement) that
returns anyarray. That actually works how I hope:

paul=# CREATE OR REPLACE FUNCTION range_agg3(anyelement)
RETURNS anyarray
AS $$
BEGIN
RETURN ARRAY[$1];
END;
$$
LANGUAGE plpgsql;

paul=# select range_agg3(daterange('2016-05-04', '2016-07-01'));
range_agg3
-----------------------------
{"[2016-05-04,2016-07-01)"}

But of course that is not as restricted as I'd like. Are there any
better options?

Thanks,
Paul