Reiter’s logic for default reasoning
A default rule has the form
where a, b1, b2, ..., bn, and g are sentences.
Here a is the prerequisite of the default rule,
b1, b2, ..., bn are the justifications of the rule, and
g is the consequent of the default rule
A theory is a pair of sets (F, D), where F is a set of sentences and D a set of default rules