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' repeat_inside('aababcc') == 'abab' repeat_inside('abc') == '' repeat_inside('abcabcabab') == 'abcabc'