dag is a pic or POSTSCRIPT preprocessor that draws directed graphs. It works well on acyclic graphs and other graphs that can be drawn as hierarchies. Graph descriptions contain nodes, edges, and optional control statements. Here is a drawing of a graph from Forrester's book, World Dynamics (Wright-Allen, Cambridge, MA, 1971). It took 2.1 CPU seconds on a VAX-8650 to make this drawing. S8 9 S24 25 27 S1 2 10 S35 43 36 S30 31 33 42 T1 26 T24 3 16 17 18 11 14 13 12 32 T30 34 4 15 19 29 37 39 41 38 40 23 5 21 20 28 6 T35 22 7 T8 I. Introduction Directed graphs have many applications in computing, such as describing data structures, finite automata, data flow, procedure calls, and software configuration dependencies. A picture is a good way to represent a directed graph. It is seldom easy to understand much about a graph from a list of edges, but with a picture one can quickly find individual nodes, groups of related nodes, and trace paths in the graph. The main obstacle is that it can be...