Solution (source code)

= Solution

The <conditional probability> product rule gives $P(A\mid B)P(B)=P(A\cap B)$. Applying it once more gives
$$
\boxed{P(A\mid B)P(B)P(C\mid A\cap B)=P(A\cap B\cap C).}
$$
The conditionals in the original expression require their conditioning events to have positive probability.