input: 
  from typing import List, Union
  
  def find_pair(lst: List[int], target: int) -> Union[List[int], None]:
      """ Find a pair of numbers from 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, 4, 5, 6], 12)
      >>> find_pair([5, 5, 5, 5], 10)
      [5, 5]
      """
      for i in range(len(lst)):
          if (target - lst[i]) in lst[i+1:]:
              return [lst[i], target-lst[i]]
      return None
  
output:     for i in range(len(lst)):
          for j in range(i + 1, len(lst)):
              if lst[i] + lst[j] == target:
                  return [lst[i], lst[j]]
      return None