Some functions were defined for the query layer, and coded in embryonic form, which were not in the end used within the interface developed for the current project. For example, the query model includes the general definition of a document set, which has been used to implement hit-lists and ``pool'' document lists. For certain purposes (as discussed in Appendix A, and in section 3.5.1 of the following chapter) it may be required to merge two or more document sets, or to use one as a filter for another. Members of a merged set should retain information about their source, so that, for example, it would be possible to show what documents in a new hit-list were also present in earlier hit-lists, and whether their new rank is higher or lower than it was previously.
An original objective for ENQUIRE was the creation of a configurable user interface, but the word ``configurable'' can be variously interpreted. The TCL scripting language proved to be a very flexible tool for modifying the interface in the light of the formative evaluations, and it was easy to create minor variations in the script to cater for the different record structures in the LBS and INSPEC databases. But the potential exists for other forms of configurability.
At a superficial level for example, some parameters like colours or fonts could be made selectable at execution-time --- perhaps even by the user. More serious variations in both appearance and functionality could be incorporated, and exploited by researchers undertaking comparative studies. Total generalisation would involve the definition of a high-level command language, invoking TCL procedures to specify any combination of databases, search methods and interface options needed for a particular experiment.