input: Write a function to remove duplicates from a list and return the list sorted in ascending order. The function should preserve the order of the first occurrence of each element. 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([4, 4, 4, 4])==[4]
  assert remove_duplicates([1, 1, 2, 2, 3, 3])==[1, 2, 3]
  assert remove_duplicates([3, 1, 2, 1, 4, 3])==[3, 1, 2, 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 sorted(result)