<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def append_value(self, valuename, valuenum=None):
 """Append another enumeration value to the `enum`. The numeric value may be None in which case it is automatically determined by increasing the value of the last item. When the 'values' attribute is accessed the resulting list will be in the same order as append_value() was called. :param valuename: The name of the value. :type valuename: str :param valuenum: The numeric value or None. :type valuenum: int """

        # No number given? Then use the previous one + 1
        if valuenum is None:
            if not self._values:
                valuenum = 0
            else:
                valuenum = self._values[-1][1] + 1

        # Store the new value
        self._values.append((valuename, int(valuenum)))