<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _string_to_int(self, s):
 """Read an integer in s, in Little Indian. """

        base = len(self.alphabet)
        return sum((self._letter_to_int(l) * base**lsb 
                    for lsb, l in enumerate(s)
                   ))