use $\bullet n$ and $n \bullet$ to denote the set of inputs and outputs of n respectively. Let $C(s,e)$ indicate the number of occurrences (count) of element $e$ in multiset $s$. A transition $t\in T$ is enabled in a marking $M$ of net $N$ if $\forall p \in \bullet t : C(M,p)>0$. An enabled transition $t$ may fire, removing one token from each of the input places $\bullet t$ and producing one token for each of the output places $t\bullet$.

Figure \[fig:double\_flower\] shows three Petri nets, with the circles representing places, the squares representing transitions. The black squares represent invisible transitions, or, $\tau$ transitions. Places annotated with an **f** belong to the final marking, indicating that the process execution can terminate in this marking.

The topmost Petri net in Figure \[fig:double\_flower\] initially has one token in the place $p1$, indicated by the dot. Firing of silent transition $t1$ takes the token from $p1$ and puts a token in both $p2$ and $p3$, enabling both $t2$ and $t3$. When $t2$ fires, it takes the token from $p2$ and puts a token in $p4$. When $t3$ fires, it takes the token from $p3$ and puts a token in $p5$. After $t2$ and 