In order to generalize the notion of "current node" to such situations we define the State of a hypertext as a boolean function S on the objects of the hyperdocument. The value true means that the object is accessible (or visible, or current), while false means that the object is not accessible.
We only consider accessibility for single node-views, i.e. for individual views (node tower objects) in a city. When a node is accessible we assume that all the anchors to which it is connected are also accessible.
The basic operation for hypertext: follow link is defined as a function from states to states. A link l = (i, s, d, v) is enabled when S(n) = true for some node n to which the source anchor s is connected. The result of following an enabled link l is a new state S':
Using this definition the behavior of a hypertext can be represented by means of Boolean Petri-net. All other operations in the Tower Model are also expressed in the Petri-net formalism. Boolean Petri-nets can easily be translated to standard Petri-nets. Thus, the analysis techniques for Petri-nets can be used to find potential browsing problems with hyperdocuments, such as small loops and dead ends.