input: Write a function to find the start index of a substring in a given string. If the substring is not found in the string, return -1. If the substring is an empty string, return 0.
  def find_substring(source, substring):
      '''This function returns the start index of a substring in a given string '''
      if substring == '':
          return 0
      for i in range(len(source) - len(substring) + 1):
          if source[i:i + len(substring)] == substring:
              return i
      return -1
  assert find_substring("hello world", "world")==6
  assert find_substring("hello world", "o")==4
  assert find_substring("hello world", "planet")==-1
  assert find_substring("hello world", "")==0
  assert find_substring("hello world", "l")==2
output: def find_substring(source, substring):
      if substring == '':
          return 0
      for i in range(len(source) - len(substring) + 1):
          if source[i:i + len(substring)] == substring:
              return i
      return -1