[plt-scheme] How to change the following code to make it handle nested lists?

At the risk of "me-too"-ism, you need to start by revising your data  
definition to include nested lists of numbers.  Once you've done that,  
clearly and unambiguously, the structure of a function to work on such  
lists should fall out as naturally as did the flat-list-processing  
structure.

Stephen Bloch
sbloch at adelphi.edu