code: def get_remainder_set(self, j):
          
          result = []
          ancestors = self.get_update_set(j)
  
          # This runs in O(log(N)log(N)) where N is the number of qubits.
          for a in ancestors:
              for c in a.children:
                  if c.index < j:
                      result.append(c)
  
          return result
docs: Return the set of children with indices less than j of all ancestors
          of j. The set C from (arXiv:1701.07072).
  
          :param int j: fermionic site index
  
          :return: children of j-ancestors, with indices less than j
          :rtype: list(FenwickNode)