input: 
  from typing import List, Optional
  
  def find_pair(lst: List[int], target: int) -> Optional[List[int]]:
      """ Find a pair of numbers in the given list that add up to the given target.
      >>> find_pair([1, 2, 3, 4, 4, 5, 6], 8)
      [2, 6]
      >>> find_pair([1, 2, 3, 4, 5], 10)
      None
      """
      seen = set()
      for num in lst:
          if target - num in seen:
              return [target - num, num]
          seen.add(num)
      return None
  
  find_pair([1, 2, 3, 4, 4, 5, 6], 8)
  
output:     seen = set()
      for num in lst:
          if target - num in seen:
              return [target - num, num]
          seen.add(num)
      return None