Exercise 2.86.  Suppose we want to handle complex numbers whose real 
parts, imaginary parts, magnitudes, and angles can be either ordinary 
numbers, rational numbers, or other numbers we might wish to add to the 
system. Describe and implement the changes to the system needed to 
accommodate this. You will have to define operations such as sine and 
cosine that are generic over ordinary numbers and rational numbers.

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All of the mathematical operations done by the complex number package 
will need to be changed to use the apply-generic versions rather than 
the native Scheme operations.

These operations need to be available for all number types that can be 
used as complex number parts.

These are: square, +, *, all of which are already implemented, and sine 
and cosine which need to be added to the system.

I am not interested in actually doing the implementation, but it seems 
straightforward.