Try to develop your application using a well-defined set of packages. Particularly, try not to intern symbols in built-in packages. You may find at delivery time that a particular built-in package is suitable for throwing out, and therefore have to go back and take your symbol out of it in order to do so safely.
Warning: When you use built-in packages in your own packages (via defpackage ), take care when naming symbols, since they may accidentally tie up with external function or class definitions in the built-in package and cause them to be retained unnecessarily. (This retention occurs because Delivery does not throw out unused definitions if they are referred to by some other symbol in the application -- See Use of symbols, functions, and classes.)