I'm reading a book that provides a bit of back ground of graph theory. There are a couple of definitions I don't understand.
A data-flow graph $G_d(V,E)$ is a directed graph whose vertex set $V = \left\{ v_i \;;\; i = 1,2,...,n_{ops} \right\}$ is in one-to-one correspondence with the set of tasks.
Later the following definition is given
A sequencing graph $G_s(V,E)$ is a hierarchy of directed graphs. A generic element in the hierarchy is called sequencing graph entity
It then specifies
A sequencing graph is an extended data-flow graph that has two kinds of vertices: operations and links, the latter linking other sequencing graph entities in the hierarchy.
An example is provided
Let us consider first a sequencing graph entity that has only operation vertices, e.g., a non-hierarchical model or an entity that is a leaf of the hierarchi. The vertx set $V$ is in one-to-one correspondence with the operations. The edge set $E$ models the dependencies due to data flow or serialization. The graph has two major properties. First, it is acyclic...
However a Dataflow graph has to be directed, and there are many directed graphs that can have cicles, right?
Am I missing something?