Non-convexity is important under oligopolies and especially monopolies. One of the key elements deregulation sought to end were airline oligopolies and monopolies. Monopolies and oligopolies are often accused of, and sometimes found guilty of, anti-competitive practices. In their view, this would bypass nation-states, corporate oligopolies, ideological NGOs, cults and mafias. When looking at oligopolies the problem of "interdependence" arises. Oil companies, grocery stores or even telecommunication companies are examples of oligopolies. In most countries, this system of licensing makes many markets local oligopolies. Alternatively, oligopolies can be fiercely competitive and engage in flamboyant advertising campaigns. Oligopolies have their own market structure. Later, some market models were built using game theory, particularly regarding oligopolies. Oligopolies differ from price takers in that they do not have a supply curve. In relation to customary price points, oligopolies can also generate price points.