code: def non_decreasing_series(n, size):
      '''Lists all combinations of 0,...,n-1 in increasing order'''
  
      if size == 1:
          return [[a] for a in range(n)]
      else:
          lc = non_decreasing_series(n, size-1)
          ll = []
          for l in lc:
              last = l[-1]
              for i in range(last, n):
                  e = l + [i]
                  ll.append(e)
          return lf non_decreasing_series(n, size):
      '''Lists all combinations of 0,...,n-1 in increasing order'''
  
      if size == 1:
          return [[a] for a in range(n)]
      else:
          lc = non_decreasing_series(n, size-1)
          ll = []
          for l in lc:
              last = l[-1]
              for i in range(last, n):
                  e = l + [i]
                  ll.append(e)
          return ll
docs: Lists all combinations of 0,...,n-1 in increasing order