It is important to note that for Truth Maintenance (and logical assertions) to work at all, your Fact objects (which may be Javabeans) must override equals and hashCode methods (from java.lang.Object) correctly. As the truth maintenance system needs to know when 2 different physical objects are equal in value, BOTH equals and hashCode must be overridden correctly, as per the Java standard.

Two objects are equal if and only if their equals methods return true for each other and if their hashCode methods return the same values. See the Java API for more details (but do keep in mind you MUST override both equals and hashCode).