Using Boolean functions in SQL*PLUS

Is it possible to use a boolean function call in a SQL*PLUS routine. For example:

       select x,y,z
       from dual 
       where function = 'TRUE';

When I try to do this I get the message:
     ORA-06571: Function ZFDIFUND does not guarantee not to update database

Is my syntax wrong or is it just not possible to use Boolean functions like this. Any help would be appreciated. Thanks. Received on Thu Dec 07 1995 - 00:00:00 CET