What does the notation $\nrightarrow$ mean in the following definition of the map function:
Definition 2. A workflow specification $S$ is a tuple $(Q,top,T^\diamond,map)$ such that:
$Q$ is a set of EWF-nets,
$top \in Q$ is the top level workflow,
$T^\diamond = \bigcup_{N\in Q}T_N$ is the set of all tasks,
$\forall_{N_1,N_2\in Q} N_1\neq N_2 \implies (C_{N_1}\cup T_{N_1})\cap (C_{N_2} \cup T_{N_2})=\emptyset$, i.e. no name clashes
$map : T^\diamond \nrightarrow Q/\{top\}$ is a surjective injective function which maps each composite task onto an EWF net, and
the relation $\{(N_1,N_2)\in Q \times Q \,|\, \exists_{t\in dom(map_{N_1})} map _{N_1}(t)=N_2\}$ is a tree