filling the collection in pl sql

There is a function that, in theory, should fill the collection with data from any table. The input is a query string and a row of columns framed with tags to be displayed. For example:

select f1('select * from a', '<#ID#>') from dual 

Here is the function itself:

 CREATE OR REPLACE FUNCTION f1(QUERY IN VARCHAR2, VALUESES VARCHAR2) RETURN VARCHAR2 IS cur PLS_INTEGER := DBMS_SQL.OPEN_CURSOR; cols DBMS_SQL.DESC_TAB; ncols PLS_INTEGER; answer VARCHAR2(30000); parsing varchar2(30000); TYPE pol is table of number; pol1 pol; BEGIN DBMS_SQL.PARSE (cur, query, DBMS_SQL.NATIVE); DBMS_SQL.DESCRIBE_COLUMNS (cur, ncols, cols); FOR colind IN 1 .. ncols LOOP parsing := '<#'||cols (colind).col_name||'#>'; if parsing = regexp_substr(VALUESES, '<#[[:alpha:]]+#>') then pol1 := set(dbms_sql.column_value(cur, ncols, pol1)); end if; END LOOP; DBMS_SQL.CLOSE_CURSOR (cur); return answer; END; 

But nothing comes out. How to add data to the collection? Well, then how to pick up the data from there to return the answer?