public boolean checkArc(Arc paArc, Graph graph) {

        Element p1 = paArc.getOutElement();
        Element p2 = paArc.getInElement();
        // Petri Net - Place / Transition
        if (graph instanceof PetriNet) {
            if (((p1 instanceof AbsPlace) && (p2 instanceof Transition)) | ((p2 instanceof AbsPlace) && (p1 instanceof Transition))) {
                return true;
            }
        }
        // Precedence Graph - Node / Node
        if (graph instanceof PrecedenceGraph) {
            if (((p1 instanceof Node) && (p2 instanceof Node)) && !p1.equals(p2)) {
                return true;
            }
        }
        return false;
    }