QPE: Using assumption-based truth maintenance for qualitative simulation

Efficient qualitative simulators are crucial to continued progress in qualitative physics . Assumption-based truth maintenance systems (ATMS) were developed in part to simplify writin g such programs. This paper identifies several abstractions for organizing ATMS-based problem solvers which are especially useful for envisioning . In particular, we describe the many-world s database, which avoids complex temporal reference schemes ; how to organize problem-solvin g into justify/assume/interpret cycles which successively construct and extend partial solutions ; and closed-world tables, which provide a mechanism for making closed-world assumptions . We sketch the design of the Qualitative Process Engine, QPE, an implementation of Qualitative Proces s theory, to illustrate the utility of these abstractions . On the basis of our experience in developin g QPE and analysing its performance, we draw some general conclusions about the advantages an d disadvantages of assumption-based truth maintenance systems .