e will demand that:
|
|    |_| (n = 1 to oo) A_n  is in  $A$
|
| and
|
|    |^| (n = 1 to oo) A_n  is in  $A$.
|
| Here we are using the shorthand notation:
|
|    |_| (i = 1 to oo) A_i  =  A_1 |_| A_2 |_| ...
|
| to denote the union of all the sets of the sequence, and:
|
|    |^| (i = 1 to oo) A_i  =  A_1 |^| A_2 |^| ...
|
| to denote the intersection of all the sets of the sequence.
|
| A collection of subsets of a given set !W! that is closed
| under countable set theory operations is called a !s!-field