Character escape in "CREATE FUNCTION ..."

Dear All,
I am trying to write a function in Postgresql, which takes 2 floats and
returns a box. But seems the nested single-quote in the AS clause
prevent $1 and $2 from being expanded. Besides writing a C function
instead of a SQL one, is there any way to solve this problem?

Thanks a lot.
-Stan

CREATE OR REPLACE FUNCTION func_radec_to_box(float, float) RETURNS box
AS 'SELECT box \'(($1, $2), (1.3, 1.4))\''
LANGUAGE 'sql'
WITH (ISCACHABLE);

Error message:
psql:func_radec_to_box.sql:4: ERROR: Bad box external representation
'(($1, $2), (1.3, 1.4))'