Details for Wednesday Morning

  An LON is empty or (cons NUMBER LON). What's unusual about such a definition? 
  Think of a LON as an inventory price list. 
  Examples of LONs. 
  Let's apply our question-and-answer game to get as far as possible. 
  Two functions: 
    ;; sum : LON -> num 
    ;; how-many : LON -> num

  The DESIGN RECIPE FOR REC DATA: _self-references_ in data definitions
  imply similar self-references in program definitions

  Slogan: "recursion in programs for recursion in data"

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  An LOS is empty or (cons SYMBOL LOS). 

   ;; contains-a : LOS -> bool

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  What other example of recursive data do we know (since the age of 5)? 

  A -natural number is either  or (+ n 1)
  Examples. 

  A natural number is either 0 or (add1 0). 

  A function: 
    ;; factorial : NatNum -> Num 
    ;; triangle : NatNum -> Num
    ;; hellos :  NatNum -> LOS

RECAP: 
  DESIGN RECIPE: list of symbols; member-mary : LOS -> bool
  when we design a function, ask - which design recipe
  when we design an expression, ask - re-use of function

ASSIGNMENTS:
  - 9.7; 9.8; 9.10 

  Yet another example:  LON; eliminate0 : LON -> LON

  - 11.3; 11.5