s = 'abcdefgfedcba'
print(s)
for i in range(6, 0, -1):
    s = s.replace(s[i], '_')
    print(s)

b7f5c60272e4f7a5eacd3bab53f2cff9d91ab14b
<mask token>

<mask token>
print(s)
for i in range(6, 0, -1):
    s = s.replace(s[i], '_')
    print(s)

s = 'abcdefgfedcba'
print(s)
for i in range(6, 0, -1):
    s = s.replace(s[i], '_')
    print(s)
