input: Write a function to find elements that appear exactly once in a list.
  assert find_unique_elements([1, 2, 2, 3, 3, 4]) == [1, 4]
  assert find_unique_elements([4, 5, 5, 6, 7, 8, 8]) == [4, 6, 7]
  assert find_unique_elements([9, 9, 10, 11, 11, 12, 13]) == [10, 12, 13]
output: def find_unique_elements(lst):
      from collections import Counter
      count = Counter(lst)
      return [element for element in lst if count[element] == 1]