Exercise 2.76: As a large system with generic
operations evolves, new types of data objects or new operations may be needed.
For each of the three strategies—generic operations with explicit dispatch,
data-directed style, and message-passing-style—describe the changes that
must be made to a system in order to add new types or new operations.  Which
organization would be most appropriate for a system in which new types must
often be added?  Which would be most appropriate for a system in which new
operations must often be added?