What does it mean for a method to be optional ? 

There is considerable redundancy in OKBC operations -- some operations can be implemented in terms of others. For example, consider the operations class-p and individual-p that check whether an entity is a class or an individual. If we define either of these two operations, it can be used to define the other, because the domain of discourse is partitioned into classes and individuals. The minimal set of OKBC operations that must be implemented, also known as mandatory methods, is known as a kernel. The other methods are said  to be optional.

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