Reason Maintenance or Truth Maintenance is a technique for maintaining a propositional database that has a Retract operation. We assume that the database has three operations:

• Assert(P): Assert that the proposition P is true.

• Ask(P): Ask whether something that unifies with the proposition P is true, e.g. Ask(Loves(?x, Mary)).

• Retract(P): Remove the proposition P from the database.

We assume that the database does forward inference from assertions, so that if P and P &rarr Q have been asserted, Q will also be asserted.

For efficiency, the database may be indexed for fast lookup on predicates and ground terms as arguments of the predicates.

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