input: Write a python function to find the longest common starting substring between elements of two sets.
  assert find_longest_common_starting_substring(set(["pineapple" ,"mango","apple","orange"]), set(["grandfather","grandmother","father","mother"])) == ''
  assert find_longest_common_starting_substring(set(["apple","apricot","banana"]), set(["appliance","application","banana"])) == 'appl'
  assert find_longest_common_starting_substring(set(["hello","world"]), set(["hellenic","helicopter"])) == 'hel'
output: def find_longest_common_starting_substring(set1, set2):
      def common_prefix(str1, str2):
          min_length = min(len(str1), len(str2))
          for i in range(min_length):
              if str1[i] != str2[i]:
                  return str1[:i]
          return str1[:min_length]
  
      longest_common_substring = ''
      for word1 in set1:
          for word2 in set2:
              prefix = common_prefix(word1, word2)
              if len(prefix) > len(longest_common_substring):
                  longest_common_substring = prefix
  
      return longest_common_substring