How to remove Oracle text preference found in CTX_USER_PREFERENCES?

 I'm using 11g db on linux 4.  In my sql scripts, I can determine preferences previously created as they appear in the CTX_USER_PREFERENCES view.  However, I cannot find a way to drop this so I can create another one.  Usually, I use 'EXEC CTX_DDL.DROP_PREFERENCE('AUTO_LEXER')' when I'm in sqlplus.  But how do I implement this within a procedure where I can first check if this preference exists and if yes, then drop it.

Thanks in advance.