IGEN in some ways resembles a blackboard architecture (Nii 1986b(Nii , 1986a; like a blackboard, its workspace contains several possible utterances that are described at different levels of structure and are built up incrementally by various independent components.
it doesn't have the strict hierarchical structure that usually characterizes blackboard systems; each request from the planner may correspond to several linguistic options, and the linguistic options may handle (parts of) more than one request.