Since 23 is missing, it is natural to wonder:
Is anything else missing?
Answer: yes -- D= {23, 139, 211, 422, 461, and 761}
However, except for a few, it seems that all natural numbers are in the tree.
In fact, the statement that all natural numbers other than those in D and less than N is true for increasingly large numbers N:
| N | year | machine | place |
| 150 | 1976 | pencil | U-Colorado |
| 5000 | 1978 | Xerox S5 | U-Wyoming |
| 6000 | 1980 | Xerox S6 | U-Tulsa |
| 60,000 | 1984 | Vax11-750 | U-Alaska |
| 360,000 | 1991 | Mac IIsi | Williams C |