How to execute 'set session role' from plpgsql function?

Hi,everybody!

I wish to have a function with code above, but compiller generate
syntactic error at the line "SET SESSION ROLE wishedrole;".

How to pass the wishedrole value to the structure?

CREATE OR REPLACE FUNCTION f_switch_role(INOUT text,INOUT boolean) AS
$BODY$
DECLARE
wishedrole ALIAS FOR $1;
resetrole ALIAS FOR $2;
BEGIN
if resetrole=true then
RESET ROLE;
RETURN;
end if;

>>>>ERROR OCURS AT THE NEXT LINE <<<<<<
SET SESSION ROLE wishedrole;
RETURN;

END;
$BODY$
LANGUAGE 'plpgsql' VOLATILE