fcn topoSort(data){ // data is L( L(root,L(leaves)),...)
   allDs:=data.pump(List,fcn(rds){ T(Void.Write,Void.Write,rds[1]) }).copy();
   roots:=Dictionary(data); // dictionary of root:leaves
   L:=List();
   S:=data.pump(List,'wrap([(r,_)]){ if(allDs.holds(r)) Void.Skip else r }).copy();
   while(S){        //while S is non-empty do
      (n:=S.pop()) : L.append(_); //remove a node n from S, add n to tail of L
      foreach m in (ds:=roots.find(n,List)){ //node m with an edge e from n to m
	 allDs.del(allDs.index(m));
	 if (Void==allDs.find(m)) S.append(m); //m has no other incoming edges
      } roots.del(n);  // remove edge e from the graph
   }
   if(roots) throw(Exception.ValueError("Cycle: "+roots.keys));
   L
}