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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def invalidate_config_var_entry(self, index):
 """Mark a config variable as invalid."""


        if index == 0 or index > len(self.config_database.entries):
            return [Error.INVALID_ARRAY_KEY, b'']

        entry = self.config_database.entries[index - 1]
        if not entry.valid:
            return [ConfigDatabaseError.OBSOLETE_ENTRY, b'']

        entry.valid = False
        return [Error.NO_ERROR]