The collection classes support a very rich protocol--so rich that at times it can be confusing. To help structure the following section, the method names are grouped in loosely defined functional categories. The protocol categories are described in the following section, and the table shows a cross-reference mapping between classes and the protocols they support.

A difficulty arises in cases where two or more classes support the same message name, but the semantics of the message are not identical. Usually, one message is just a refinement of the other, that is, it imposes additional restrictions on the implementation of the message. A more serious problem arises when the messages have completely different meanings but share the same name. This document will identify such cases explicitly.