Definition 11.1.5 (Algebra).label Let $X$ be a nonempty set and $\cali{A}\suf 2^{X}$ satisfying:

1. (1)

   $\cal{A}\neq\emp$

2. (2)

   For any $E_{1},...,E_{n}\in \cali{A}$, $\cups{n}{i=1}E_{i}\in \cal{A}$

3. (3)

   For each $E\in \cal{A}$, $E^{c}\in\cal{A}$