docs: def get_nth_bell_number(n):
      """
      Returns the (num_items - 1)-th Bell number using recursion.
      The Bell numbers count the number of partitions of a set.
  
      Args:
          n (int): number of items in a set
  
      Returns:
          int:
  
      References:
          http://adorio-research.org/wordpress/?p=11460
  
      CommandLine:
          python -m utool.util_alg --exec-bell --show
  
      Example:
          >>> # DISABLE_DOCTEST
          >>> from utool.util_alg import *  # NOQA
          >>> n = 3
          >>> result = get_nth_bell_number(n)
          >>> print(result)
          5
      """
      import utool as ut
      import scipy.special
      @ut.memoize
      def bell_(n):
          if n < 2:
              return 1
          sum_ = 0
          for k in range(1, n + 1):
              sum_ = sum_ + scipy.special.binom(n - 1, k - 1) * bell_(k - 1)
          return sum_
      nth_bell = bell_(n)
      return nth_bell