input: 
  def longest_common_substring(str1: str, str2: str) -> str:
      """
      Find the longest common substring between two strings.
  
      >>> longest_common_substring("Helwallomst", "Welcome")
      'omst'
      """
      match = 0
      substr = ""
      for i in range(len(str1)):
          for j in range(len(str2)):
              l = 0
              while (i + l < len(str1) and j + l < len(str2) and str1[i + l] == str2[j + l]):
                  l += 1
              if l > match:
                  substr = str1[i:i + l]
                  match = l
      return substr
  
output:     match = 0
      substr = ""
      for i in range(len(str1)):
          for j in range(len(str2)):
              l = 0
              while (i + l < len(str1) and j + l < len(str2) and str1[i + l] == str2[j + l]):
                  l += 1
              if l > match:
                  substr = str1[i:i + l]
                  match = l
      return substr