code: def find_nth(haystack, needle, n):
      """
      Returns the starting index of the nth occurrence of the substring 'needle' in the string 'haystack'.
      """
      start = haystack.find(needle)
      while start >= 0 and n > 1:
          start = haystack.find(needle, start + len(needle))
          n -= 1
      return start
  
label_desc: Use image node_id 1 for calling a global function with example usage: find_nth(haystack, needle, n) and returns: start