input: Write a function to remove duplicates from a list. The function should take a list as input and return a new list containing only the unique elements in the order they first appear. If the input list is empty, the function should return an empty list.
  assert remove_duplicates([1, 2, 3, 4, 3, 2, 1])==[1, 2, 3, 4]
  assert remove_duplicates([5, 6, 7, 8, 7, 5, 6])==[5, 6, 7, 8]
  assert remove_duplicates([1, 2, 2, 3, 3, 3, 4, 4, 4, 4])==[1, 2, 3, 4]
  assert remove_duplicates([])==[]
output: def remove_duplicates(lst):
      seen = set()
      result = []
      for item in lst:
          if item not in seen:
              seen.add(item)
              result.append(item)
      return result