End-user modifiability allows different users to tailor a system in order to pursue different tasks, to have different preferences, and to adapt it to changing needs over time. In knowledge-based systems, end-user modifiability can help eliminate the knowledge acquisition bottleneck: domain experts can extend the knowledge base in such systems without having to rely on knowledge engineers. Experiments with end users have led to the identification of several principles that must be observed in

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... der to make systems end-user modifiable. These principles have been followed in the implementation of MODIFIER, an end-user modification component for design environments, and NETWORK, a design environment for local area networks.