text.mod/graphviz.mod/graphviz/cmd/gvpr/lib/cliptree

/* Construct subgraph reachable from node ARGV[0] by forward edges */
BEG_G {
  node_t r = node($,ARGV[0]);

  $tvroot = r;
  $tvtype = TV_fwd;
}
N{$tvroot=NULL; subnode($T,$);}