
| of subsets of !W!.  (The !s! [sigma] is put in to distinguish
| such a collection from a field of subsets.)  More formally we
| have the following:
|
| Definition 1.
|
| A nonempty collection of subsets $A$ of a set !W!
| is called a !s!-field of subsets of !W! provided
| that the following two properties hold:
|
|    1.  If A is in $A$, then A^c is also in $A$.
|
|    2.  If A_n is in $A$, n = 1, 2, ..., then:
|
|           |_| (i = 1 to oo) A_i
|
|        and
|
|           |^| (i = 1 to oo) A_i
|
|    