[T]he hierarchy of mathematics competences does not follow a total order organization, as the theory of stages unfortunately suggests, but rather a partial order one: situations and problems that students master progressively, procedures and symbolic representations they use, from the age of 2 or 3 up to adulthood and professional training, are better described by a partial-order scheme in which one finds competences that do not rely upon each other, although they may all require a set of more primitive competences and [may] all be required for a set of more complex ones.