A numbering of the vertices of special networks Original Research Article

We characterize acyclic digraphs which remain acyclic under a certain type of arc addition. The vertices of such digraphs D admit a labelling f such that f(x) < f(y) and f(x) = f(z) whenever (x, y) and (x, z) are arcs of D. The study of such digraphs has been initiated by a software development problem.