input: from typing import List, Optional
  
  def find_pair(lst: List[int], target: int) -> Optional[List[int]]:
      """
      Given a list of numbers and a target number, find a pair of numbers from the given list
      that add up to the given target. If such a pair exists, return them as a list in the order
      they appear in the input list. If no such pair exists, return None.
      
      >>> find_pair([1, 2, 3, 4, 4, 5, 6], 8)
      [2, 6]
      >>> find_pair([1, 2, 3, 4, 5], 10)
      None
      """
output:     seen = {}
      for num in lst:
          complement = target - num
          if complement in seen:
              return [complement, num]
          seen[num] = True
      return None