Robinson's original formulation of nonstandard analysis falls into the category of the "semantic approach". As developed by him in his papers, it is based on studying models (in particular saturated models) of a theory. Since Robinson's work first appeared, a simpler semantic approach (due to Elias Zakon) has been developed using purely set-theoretic objects called superstructures. In this approach "a model of a theory" is replaced by an object called a "superstructure" over a set . Starting from a superstructure one constructs another object using the ultrapower construction together with a mapping that satisfies the transfer principle. The map * relates formal properties of and . Moreover, it is possible to consider a simpler form of saturation called countable saturation. This simplified approach is also more suitable for use by mathematicians who are not specialists in model theory or logic.