History of Sigma algebra

	Revision 2 . . November 22, 2001 10:53 am by Iwnbap [union -> countable union (this confused me once!)]
	Revision 1 . . November 22, 2001 10:45 am by (logged).200.219.xxx [added sigma-algebra]

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Difference (from prior major revision) (no other diffs)

Changed: 7c7

3) If S1, S2, S3, ... is a sequence in X then their union must also be in X.

3) If S1, S2, S3, ... is a sequence in X then their countable union must also be in X.