This is a part of the struct module in the tcllib distribution. It is a companion package for struct::graph. It was created as part of Google's Summer Of Code 2008.
The documentation for this package will joyfully be found at http://tcllib.sourceforge.net/doc/graphop.html (Not yet)
Use this to perform a number of graph algorithms on directed graph objects providing the same API as the graphs created by struct::graph.