input: Write a function to compute the highest sum of non-adjacent subsequences in a given array.
  assert max_subseq_sum([1, 2, 9, 4, 5, 0, 4, 11, 6]) == 26
  assert max_subseq_sum([1, 2, 9, 5, 6, 0, 5, 12, 7]) == 28
  assert max_subseq_sum([1, 3, 10, 5, 6, 0, 6, 14, 21]) == 44
output: def max_subseq_sum(arr):
      if not arr:
          return 0
      if len(arr) == 1:
          return arr[0]
      
      incl = arr[0]
      excl = 0
      
      for i in arr[1:]:
          new_excl = max(incl, excl)
          incl = excl + i
          excl = new_excl
          
      return max(incl, excl)