Unofficial Introduction to Quantum Probability

The following item comes from the glossary of a popular-science book titled "Quantum Entanglement and the Computing Revolution. The Feynman Processor" by Gerard J. Milburn (Perseus Books, 1998), page 195.  

Bayes' rule: The probability for an event which can happen in two indistinguishable ways is the sum of the probability for each way considered separately.