SET ROLE in PL/SQL

We have not had any success with trying to execute the SQL command:

          SET ROLE ThisRole IDENTIFIED BY ThisPassword; using dynamic SQL in either an anonymous PL/SQL block or in a stored procedure.

Can this be done? If not, is there an explanation as to why this is not permitted?

--
========================================================
Jerry Apfelbaum
Eastern Sun Group Inc.
Toronto, Canada
------------------------------------------------------------------------

(Due to proliferation of spammers, remove 'nospam' my email address.)