in this one you should find a repeating sequence inside the substring.
I have an example for you: in a string "abababc" - "ab" is a sequence that repeats more than once, so the answer will be "ababab"
`repeat_inside('aaaaa') == 'aaaaa'
repeat_inside('aabbff') == 'aa'
Submitted by https://py.checkio.org/mission/long-repeat-inside/publications/rossras/python-3/regular-expressions-now-you-have-two-problems/ - a year ago