I am trying to wrap my head around Partially Observed Markov Decision Process (POMDP). However, I am unable to understand the application of the Bayes Rule in following equation (Step Nr. 2):

Can anyone help me understand how Bayes rule can be applied for multiple conditions like this ? or at least point me towards the right literature. Thank you.

The picture is taken from here: Acting Optimally in Partially Observable Stochastic Domains