func createSubstrings(String word) -> Array {
  gather {
    combinations(word.len+1, 2, {|i,j|
        take(word.substr(i, j-i))
    })
  }
}

func findLongestCommon(String first, String second) -> String {
    createSubstrings(first) & createSubstrings(second) -> max_by { .len }
}

say findLongestCommon("thisisatest", "testing123testing")