456: Mainline

One simple way to keep track of what has gone into a union array element is to maintain a parallel array of type-identifying integers.

  *-- Indicates that the first element in trade_pointers array 
  |   points to a union that contains a stock_trade object 
  v 
*---*---*---*---*---* 
| 0 | 0 | 1 | 1 | 0 | 
*---*---*---*---*---* 
          ^ 
          | 
          *-- Indicates that the third element in trade_pointers array  
              points to a union that contains a bond_trade object