Interactive unification is a poweful technique in grammar engineering and debugging: following a user request to attempt and unify two feature structures, LUI will call back into the back-end processing system (the LKB, for example) to have the unification executed and information about unification failure(s) recorded. This technique will often be helpful in working out why a specific (sub-)structure has not been built during parsing, i.e. often one will want to unify the feature structure(s) associated to one or more existing constituent(s) in the parse chart into the argument position(s) of a phrase structure rule.
Interactive unification in LUI is activated by means of drag and drop: click on the (sub-)structure you want to unify, hold down the mouse button, and drag it onto the target location, i.e. the (sub-)structure, typically in a different LUI window, where you want the first feature structure to unified into.
The result of interactive unification will be displaed as a new LUI AVM window, either simply showing the result of unification, if successful, or presenting an AVM annotated with failure information. When an interactive unification attempt fails, LUI can display the partial result with failure sites highlighted (in red ink) and detailed. The user can navigate between the (potentially numerous) unification failures using the left and right arrow keys.