The operations discussed in chapter 3 are generalized representations of compositional techniques whereby PC sets are derived from each other. However, not all relations between PC sets are based on derivation. This chapter and the following ones will deal with other relations, seen within a historical framework. In this chapter I will discuss the evolution of the concept of PC set equivalence.

Like the term “operation,” the term “relation” evokes the world of mathematics. In mathematics, more specifically in algebra, a relation is commonly conceived as an open sentence, designated*P*, connecting the elements of two collections,*S*and*T.*