<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def followingPrefix(prefix):
 """Returns a String that sorts just after all Strings beginning with a prefix"""

        prefixBytes = array('B', prefix)

        changeIndex = len(prefixBytes) - 1
        while (changeIndex >= 0 and prefixBytes[changeIndex] == 0xff ):
            changeIndex = changeIndex - 1;
        if(changeIndex < 0):
            return None
        newBytes = array('B', prefix[0:changeIndex + 1])
        newBytes[changeIndex] = newBytes[changeIndex] + 1
        return newBytes.tostring()