+I~have tried to stick to the usual notation except where it was too inconvenient.
+Most symbols are defined at the place where they are used for the first time.
+A~complete index of symbols with pointers to their definitions is then available
+in Appendix~\ref{notapp}. This appendix also describes the formalism of
+multigraphs and of the Ackermann's function, both of which are not defined
+consistently in the common literature.
+
+To avoid piling up too many symbols at places that speak about a~single fixed graph,
+this graph is always called~$G$, its set of vertices and edges are denoted by $V$
+and~$E$ respectively, and I~also use~$n$ for the number of its vertices and $m$~for
+the number of edges. At places where there could be a~danger of confusion, more explicit notation
+is used instead.
+
+