ssp -- model for the stable set problem
    Parameters:
        - V: set/list of nodes in the graph
        - E: set/list of edges in the graph
    Returns a model, ready to be solved.

def ssp(V,E):
    """ssp -- model for the stable set problem
    Parameters:
        - V: set/list of nodes in the graph
        - E: set/list of edges in the graph
    Returns a model, ready to be solved.
    """
    model = Model("ssp")

    x = {}
    for i in V:
        x[i] = model.addVar(vtype="B", name="x(%s)"%i)

    for (i,j) in E:
        model.addCons(x[i] + x[j] <= 1, "Edge(%s,%s)"%(i,j))

    model.setObjective(quicksum(x[i] for i in V), "maximize")

    model.data = x
    return model