Write a method digit_string(self) that creates and returns a string of digits that corresponds to the current contents of the called Board objects tiles attribute. For example:

>>> b = Board('142358607') >>> b.digit_string() '142358607' 

Note that this call to digit_string() returns the string of digits that was used when creating the Board. However, this wont always be the case, because the string returned by digit_string() should reflect any changes that have been made to the tiles. For example, consider this continuation of the above example: