docs: def longest_common_substring(s1, s2):
      """
      References:
          # https://en.wikibooks.org/wiki/Algorithm_Implementation/Strings/Longest_common_substring#Python2
      """
      m = [[0] * (1 + len(s2)) for i in range(1 + len(s1))]
      longest, x_longest = 0, 0
      for x in range(1, 1 + len(s1)):
          for y in range(1, 1 + len(s2)):
              if s1[x - 1] == s2[y - 1]:
                  m[x][y] = m[x - 1][y - 1] + 1
                  if m[x][y] > longest:
                      longest = m[x][y]
                      x_longest = x
              else:
                  m[x][y] = 0
      return s1[x_longest - longest: x_longest]