Keywords: class

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### The Rope to Hang Yourself

CLASS makes use of SIC variables to allow more flexibility in the processing, in particular in procedures. SIC variables are extremely powerful, with the side effect that if you want, you can corrupt your data by overwriting some information. CLASS attempts to prevent the most disastrous errors by defining some of the most critical variables as `READONLY`. They cannot be overwritten by the user, but their values can be used in expressions, either arithmetic or logical. However, an unprotected mode is available for specific processing using the command `SET VARIABLE`.