def buttonClicked(self, symbol):
        # Clear
        if symbol == 'C':
            self.expression = self.defaultExpression
            return self.expression
        # Check the expression is currently valid
        if self.expression == self.errorExpression:
            return self.expression
        # Evaluate the expression
        if symbol == '=':
            try:
                self.expression = unicode(eval(self.expression))
            except ZeroDivisionError:
                self.expression = self.errorExpression
            return self.expression
        # Replace of add to the expression
        if self.expression == self.defaultExpression:
            self.expression = symbol
        else:
            self.expression += symbol
        return self.expression
GET last document number for this instance
dataset/ETHPy150Open twisted/nevow/examples/athenademo/calculator.py/Calculator.buttonClicked--crossover with--dataset/ETHPy150Open adlibre/Adlibre-DMS/adlibre_dms/apps/core/models.py/DocumentTypeRule.get_last_document_number