bsets' of !W!.
| It therefore seems we should demand that $A$ be a field of subsets.
| It turns out, however, that for certain mathematical reasons just
| taking $A$ to be a field of subsets of !W! is insufficient.
| What we will actually demand of the collection $A$ is
| more stringent.  We will demand that $A$ be closed
| not only under finite set theoretic operations
| but under countably infinite set theoretic
| operations as well.  In other words if
| {A_n}, n &gt;= 1, is a sequence of sets
| in $A$, w