PL/SQL: Nested conditional compilation

alter session set plsql_ccflags = 'flag_xyz:true';

create or replace procedure tq84_test_nested_preproc authid definer as
begin

   $if true $then 

      $if $$flag_xyz $then
          dbms_output.put_line('This line is printed if flag xyz is set');
      $else
          dbms_output.put_line('This line is printed if flag xyz is not set');
      $end

   $else

      $if $$flag_xyz $then
          dbms_output.put_line('This line is not printed');
      $else
          dbms_output.put_line('This line is not printed');
      $end

   $end

end tq84_test_nested_preproc;
/

begin
   tq84_test_nested_preproc;
end;
/

alter session set plsql_ccflags = 'flag_xyz:false';
alter procedure tq84_test_nested_preproc compile;

begin
   tq84_test_nested_preproc;
end;
/