 Other analytic solution concepts exist, as well. Swinkels (1992) introduced the idea of an equilibrium evolutionarily stable set, providing a further refinement of the idea of an evolutionarily stable set. (Every evolutionarily stable set contained some equilibrium evolutionarily stable set, but not every equilibrium evolutionarily stable set was an evolutionarily stable set.) Hence we see that the search for static solution concepts which suffice for capturing the idea of evolutionary stability encounters a problem structurally similar to that of the equilibrium selection problem in traditional game theory: there are multiple competing concepts of evolutionary stability, all of which have some intuitive claim to plausibility.