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1. In an aggregated structure, it is desirable for all messages to come in through the main class (the class which is the aggregated structure).  This class can then relay the incoming message to the appropriate part.  The advantage of this is that the sender of the message does not need to know anything about the part-structure, only the aggregated class.  This is known as information hiding.   The relaying of the incoming message is of course message passthrough, but is acceptable because the disadvantages of message passthrough are outweighed by the advantages of aggregation and information hiding.

2. The Team Producer model
3. Answer 2- Communicational cohesion