There is a significant difference between association and aggregation. For aggregation, a whole cannot be its own part - forming a cyclic relationship. For example, if both Student and Course objects have a reference of each other, aggregation cannot be applied here.
To clear the confusion, I e-mailed the author of The Object Primer, Scott Ambler, for clarification. He said that aggregation is back in UML 2.0 due to significant complaining within the community. Thanks.
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