Finding the type defined by TYPE_SUBNAME in ALL_ARGUMENTS

Question

I'm trying to find everything I can about the arguments to stored procedures in order to execute them and process their output.
For the argument arg_table, ALL_ARGUMENTS shows it has a TYPE_SUBNAME of arg_type_table. How can I find out that it actually contains A_COL NUMBER and B_COL NUMBER?
(I had high hopes when I found that ALL_ARGUMENTS did have the column definitions for arguments fo type arglist_cursor_type)
CREATE OR REPLACE PACKAGE testproc
IS
TYPE arg_type IS RECORD (
A_COL NUMBER,
B_COL NUMBER
type arg_table_type is TABLE OF arglist_rt;
type arglist_cursor_type is ref cursor return arglist_rt;
PROCEDURE getArgList_Table_arg (arg_table OUT arg_table_type);
END;

Answer 1

If the type is defined in an unwrapped header then you can find the source code for it in $ORACLE_HOME/rdbms/admin.
For the majority of documented built-in packages I have the definitions in Morgan's Library at www.psoug.org.
For those defined as stand-alone types you can use DESCRIBE.
For some there is no way to know and that is as Oracle wants it.