Tsort was a direct reference to Knuth's recognition and christening of topological sort as a worthy software component. This is a typical example of the interweaving of R and D which characterized the culture of Bell Labs. Builders and thinkers were acutely aware of each other, and often were two faces of one person. Grander examples may be seen in the roles that automata theory and formal languages played in Unix. (Alas, these are the subjects of projected Knuthian volumes that are still over the horizon.) Doug