o address the need for canonical abstraction, we've adopted software engineering's concept of signatures--a technique for defining software patterns or invariant procedural properties. A general model Healthcare applications have a number of additional requirements beyond the basic functions and representations that are common to many cognitive-system theories. (The "Related Work in Multiagent Healthcare Systems" sidebar describes four multiagent healthcare systems that, in different ways, illustrate these requirements.) On the basis of our experience with healthcare systems, we've identified three key requirements over and above the basic domino model: