# text_increment.py

# text_incr.py

def incr_str(s):
    p = s.rstrip('z')
    r = len(s) - len(p)
    if p: p = p[:-1] + chr(ord(p[-1]) + 1)
    else: p = 'a'
    p += 'a' * r
    return p
#

print incr_str('aaa')
print incr_str('zzz')
print incr_str('a')
print incr_str('z')
print incr_str('pizzazz')