The grammar designer can specify particular types of nodes that are not permitted to have particular types of arcs going into, or coming out of them.
For example, an integral sign never has anything to its top-left, so the graph builder is told never to build arcs so that a ``'' has something to the ``top-left'' of it. A ``+'' can never be found to the bottom left of something, so ``bottom-left'' arcs ending at a ``+'' are not built.
Currently, these restrictions are read in from an external data file. Ideally these rules should be automatically determined, at least in part, by analysing the grammar.