let named_concat lhs rhs target =
let thenode = Depgraph.find_node (cur_graph ()) target in
let count = Hashtbl.fold (fun edge _ acc -> match edge with
| InConcat(_,_) -> acc + 1
| _ -> acc) thenode.inb 0 in
if count > 0 then
raise CantAlias
else
Depgraph.add_concat (cur_graph()) lhs rhs target