It is time to put in the ability to change state variables. This requires that the user enter new equations (and be able to delete them as well). How?
I propose to put at the end of the displayed equations a final term that says something like "New Equation". If the user selects this token, then she may menu-choose only from type ReadAndWriteData. Such choice causes the insertion of the string Unspecified, Pop[whatever] into the token string, so that now the screen reads "whatever = Unspecified". From this point forward the user can select the whatever, but all the menus are grayed out and the only functional item is the "Delete" button.
Now, there is some special coding required to make this work. First, I don't want a useless token appended to all my token strings, so this token must be inserted in the routine Token2LongToken. When printed, the token must be printed as "New Equation", so perhaps that should be its label? Lastly, it must be a valid token, but I think it must be of type "Anything" and somehow we have to sense that it can only be replaced by a Pop token.