System-Z+: a formalism for reasoning with variable-strength defaults
Moisés Goldszmidt, Judea Pearl · National Conference on Artificial Intelligence · 1991
We develop a formalism for reasoning with defaults that are expressed with different levels of firmness. Necessary and sufficient conditions for consistency are established, and a unique ranking of the rules is found, called Z+, which renders models as normal as possible subject to the consistency conditions. We provide the necessary machinery for testing consistency, computing the Z+ ranking and drawing the set of plausible conclusions it entails.