Fault trees show which joint components' faults mean system faults. Fault trees can often be used to determine dependability parameters of systems. Here it is shown that i) binary decision diagrams (BDDs) can also be used to calculate system mean failure frequency, ii) modeling dynamics of fault trees does not always mean Markov modeling, iii) a deeper understanding of interrelations between s-dependent components is supported, rather, by Petri nets than by state transition graphs.