A string R is said to be a repeating prefix of string S if and only if
R + R + T = S
for some string T
For example:
"BA", "BABA", "BABABABABAT"
are all the repeating prefixes of the string "BABABABABATBABABABABATABABCD".

The LARGEST REPEATING PREFIX of a string is the repeating prefix having maximum length.

For example, "BABABABAT" is the largest repeating prefix of the ablove string.