The correct specification for flow of control is vital to the smooth running of a graphical user interface. ENQUIRE is less complex than many GUIs since it is designed as a public access system and has no setup options or menus. However as the outline of the search process in section 3.3.2 above indicates, the user has many possible options and thus the system can be in a number of different states.
To provide an overview of all the possible conditions and allow the designer to check that there were no ``blind alleys'' in the interface, a constraint matrix was produced, which is attached as Appendix B of this report. Please note that it was useful at an early stage of the development process and was not updated, so it is not a totally accurate representation of the final system.
An important point to address is that some actions are legal only in some states --- for example it is not sensible so select the ``New Search'' option before entering a query. Users need some indication of this fact; in the XOkapi GUI interface an attempt to click on a currently inactive window caused the system to produce a loud beep. For ENQUIRE it was decided to prevent anomalous actions by ``greying out'' and de-activating any option which should not be selected at that point.