Added value can be extracted from event logs generated by business processes in various ways. However, although complex computations can be performed over event logs, the result of such computations is often difficult to explain; in particular, it is hard to determine what parts of an input log actually matters in the production of that result. This paper describes how an existing log processing library, called BeepBeep, can be extended in order to provide a form of provenance: individual output events produced by a query can be precisely traced back to the data elements of the log that contribute to (i.e. "explain") the result.