To delete a variable put the name of the variable enclosed with single quotes like 'FUNC' press [ENTER] and then [TOOL] and PURGE (associated to [F6]) if you press [VAR] it is not there anymore). If you realize that you didn't want to delete the variable, right after you execute the command PURGE, you still can recover it with this procedure:
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firsrt, press [leftShift] ANS (associated with [ENTER]) and you get the object that was stored in the variable in level 2: and the variable name in level 1:
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second, at this point just press [STO], your variable is back in the directory.
Don't worry too much about using variable names predefined for special purposes, its very unlikely. I try not to work in the HOME directory, create your own directory.