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 21.6.1 Symbols as Discrete Data
 -------------------------------
 
 Numbers and symbols are similar to the extent that they both lend
 themselves to `eq?' comparison.  But symbols are more descriptive than
 numbers, because a symbol's name can be used directly to describe the
 concept for which that symbol stands.
 
    For example, imagine that you need to represent some colours in a
 computer program.  Using numbers, you would have to choose arbitrarily
 some mapping between numbers and colours, and then take care to use that
 mapping consistently:
 
      ;; 1=red, 2=green, 3=purple
 
      (if (eq? (colour-of car) 1)
          ...)
 
 You can make the mapping more explicit and the code more readable by
 defining constants:
 
      (define red 1)
      (define green 2)
      (define purple 3)
 
      (if (eq? (colour-of car) red)
          ...)
 
 But the simplest and clearest approach is not to use numbers at all, but
 symbols whose names specify the colours that they refer to:
 
      (if (eq? (colour-of car) 'red)
          ...)
 
    The descriptive advantages of symbols over numbers increase as the
 set of concepts that you want to describe grows.  Suppose that a car
 object can have other properties as well, such as whether it has or
 uses:
 
    * automatic or manual transmission
 
    * leaded or unleaded fuel
 
    * power steering (or not).
 
 Then a car's combined property set could be naturally represented and
 manipulated as a list of symbols:
 
      (properties-of car1)
      =>
      (red manual unleaded power-steering)
 
      (if (memq 'power-steering (properties-of car1))
          (display "Unfit people can drive this car.\n")
          (display "You'll need strong arms to drive this car!\n"))
      -|
      Unfit people can drive this car.
 
    Remember, the fundamental property of symbols that we are relying on
 here is that an occurrence of `'red' in one part of a program is an
 _indistinguishable_ symbol from an occurrence of `'red' in another part
 of a program; this means that symbols can usefully be compared using
 `eq?'.  At the same time, symbols have naturally descriptive names.
 This combination of efficiency and descriptive power makes them ideal
 for use as discrete data.
 

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