It is vitaly important for researchers to be able to incorporate code changes (which may be of interest personally but not appropriate for general dissemination) into newer versions of a model. It is in this way that researchers are able to take advantage of new parameterizations while retaining local personal changes. Also of importance is the ability to incorporate ``bug'' fixes. In the past, both of these operations have presented significant difficulties. These difficulties have been largely eliminated by the tools and method described in Section 3.12.