Memento is a software infrastructure to support the construction and evolution of assistant applications (or assistants) that act as adjuncts to the human mind. Each assistant embodies an e ective understanding of some information domain or problem domain. The assistant employs this understanding to aid a user or user communityin the manipulation, transmission, and storage of meaningful information artifacts from the assistant's domain, called models. Additionally, models ow via the assistant's user interface at the semantic level. Memento provides mechanismswhich address all of these common assistant responsibilities. When combined, the mechanisms result in real{time artifact{based collaboration over an assistant's models. Memento preserves the meaning of an assistant's models throughout these processes by enforcementofthe domain's membership rules, called semantic constraints. Because Memento abstracts an assistant's models and semantic constraints, it is therefore said to be domain{ independent. Memento is composed of two layers. The bottom layer, described in Chapter 2,