input: Write a function to remove all duplicate elements from a list of integers. Implement the function such that it returns a list containing only the unique elements in the order they first appear.
  assert remove_duplicates([1, 2, 3, 2, 4, 5, 6, 3])==[1, 2, 3, 4, 5, 6]
  assert remove_duplicates([5, 5, 5, 5, 5])==[5]
  assert remove_duplicates([])==[]
  assert remove_duplicates([7, 8, 9, 10, 9, 8])==[7, 8, 9, 10]
  assert remove_duplicates([1, 1, 1, 1, 2, 2, 2, 3, 3, 3])==[1, 2, 3]
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