code: def get_num_shares_on(self, on_date: datetime) -> Decimal:
          
          total_quantity = Decimal(0)
  
          accounts = self.get_holding_accounts()
          for account in accounts:
              acct_svc = AccountAggregate(self.book, account)
              quantity = acct_svc.get_balance_on(on_date)
  
              total_quantity += quantity
  
          return total_quantity
docs: Returns the number of shares for security on (and including) the given date.