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  There are times when you'd like to provide class users with an
  instance variable, but control what happens when it's
  read/changed.  The ACCESS facility lets you define a method
  that's invoked whenever the user tries to get the instance
  variable's value.  ASSIGN is the equivalent, but the method is
  invoked when the user attempts to set the instance variable.
  You can define either, neither, or both.
  
  For example:
  
       CLASS Temperature
            PROTECT  nTemp      AS NUMERIC
  
            ACCESS Celsius           INLINE self:nTemp
            ASSIGN Celsius(nNew)     INLINE self:nTemp := nNew
  
            ACCESS Fahrenheit        INLINE self:nTemp * 9 / 5 + 32
            ASSIGN Fahrenheit(nNew)                                 ;
                   INLINE self:nTemp := (nNew - 32) / 9 * 5,        ;
                          nNew
       ENDCLASS
  
  These methods are invoked automatically by code like:
  
       // accesses:
       ? "Celsius", oTemp:Celsius
       ? "Fahrenheit", oTemp:Fahrenheit
       MyFunc(oTemp:Celsius)
  
       // assigns:
       oTemp:Celsius := -40
       oTemp:Fahrenheit := -40       // same temp. as above
  
  Sometimes these "fake" instance variables are known as
  calculated or virtual instance variables.
  
  Access/assign can be very useful, for example you can start
  with a simple class providing a genuine instance variable,
  then change to access/assign without having to change the code
  that uses the class.
  
  
  An assign also provides a convenient place to put error
  checking:
  
       CLASS Temperature
            PROTECT  nTemp AS NUMERIC
  
            ACCESS Celsius      INLINE self:nTemp
            ASSIGN Celsius(nNew)
  
            ACCESS Fahrenheit   INLINE self:nTemp * 9 / 5 + 32
            ASSIGN Fahrenheit(nNew)
       ENDCLASS
  
       ASSIGN Celsius(nNew) CLASS Temperature
       if IsValid(nNew)
            self:nTemp := nNew
       else
            Error("...")
       endif
       return nNew   // return this if you want multiple assigns
                     // e.g. x := y := 40 to work as expected
  
       ASSIGN Fahrenheit(nNew) CLASS Temperature
       if IsValid(nNew)
                 self:nTemp := (nNew - 32) / 9 * 5
       else
            Error("...")
       endif
       return nNew   // return this if you want multiple assigns
                     // e.g. x := y := 40 to work as expected
  
  
  Because an access/assign is a method, you can use "super:" if
  you wish.