Blake McBride writes:
> The Lisp REP treats those specific symbols as a history mechanism
> therefore their values are dependent on how they are used in the history
> mechanism - which is system specific.
What's system-specific about these variables? From the CLHS:
| The value of ‘-’ is the form that is currently being evaluated by the
| Lisp read-eval-print loop.
| The value of ‘+’ is the last form that was evaluated, the value of
| ‘++’ is the previous value of +, and the value of ‘+++’ is the
| previous value of ++.
| The value of ‘*’ is the most recent primary value that was printed,
| the value of ‘**’ is the previous value of *, and the value of ‘***’
| is the previous value of **.
| The value of ‘/’ is a list of the most recent values that were
| printed, the value of ‘//’ is the previous value of /, and the value
| of ‘///’ is the previous value of //.