input: Write a function to determine the highest sum possible from subsequences of an array, ensuring that no two chosen elements are adjacent.
  assert max_sum_subseq([1, 2, 9, 4, 5, 0, 4, 11, 6]) == 26
  assert max_sum_subseq([1, 2, 9, 5, 6, 0, 5, 12, 7]) == 28
  assert max_sum_subseq([1, 3, 10, 5, 6, 0, 6, 14, 21]) == 44
output: def max_sum_subseq(arr):
      if not arr:
          return 0
      if len(arr) == 1:
          return arr[0]
  
      include = arr[0]
      exclude = 0
  
      for i in range(1, len(arr)):
          new_exclude = max(include, exclude)
          include = exclude + arr[i]
          exclude = new_exclude
  
      return max(include, exclude)