After you have been working for a while you may want to find out what variables you have defined (using def). The function variables will produce a listing:
> (variables) CO HC RURAL URBAN PRECIPITATION PURCHASES NIL >
If you are working on a 1Mb Macintosh you may occasionally want to free up some space by getting rid of some variables you no longer need. You can do this using the undef function:
> (undef 'co) CO > (variables) HC RURAL URBAN PRECIPITATION PURCHASES NIL >