input: Write a python function to count the number of subsequences in a given non-negative array where the product of the elements is less than k.
  assert count_subsequences([1,2,3,4], 10) == 11
  assert count_subsequences([4,8,7,2], 50) == 9
  assert count_subsequences([5,6,7,8], 15) == 4
output: def count_subsequences(arr, k):
      def count_subseq(idx, product):
          if idx == len(arr):
              return 1 if product < k else 0
          count_incl = count_subseq(idx + 1, product * arr[idx]) if product * arr[idx] < k else 0
          count_excl = count_subseq(idx + 1, product)
          return count_incl + count_excl
      
      return count_subseq(0, 1) - 1