TeachScheme! 2000

 descendant family tree -- from information to Scheme data
  pages 203-205

  PROBLEM: 
   represent descendant tree 
   ;; is-mike-descendant-of : desc-ft -> boolean

  mutually-referential data definitions 
   
  DESIGN RECIPE: adjust 
 
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  PROBLEM
   File Hierarchy: folders and files 
  
   find-a-file with name
   disk-usage of some portion 

   Go back and add a size to directories. How does du change?
   (Prepares them for extending the evaluator with new expression forms.)