Independence Extended. Three events $A, B,$ and $C$ are said to be independent if
$$
\begin{aligned}
P(A \& B) &=P(A) \cdot P(B) \\
P(A \& C) &=P(A) \cdot P(C) \\
P(B \& C) &=P(B) \cdot P(C), \text { and } \\
P(A \& B \& C) &=P(A) \cdot P(B) \cdot P(C)
\end{aligned}
$$
What is required for four events to be independent? Explain your definition in words.