A program is a text, viz. the formulation of some algorithm, expressed in a programming language. This text obeys the syntax of the programming language, and according to that syntax consists of a nested collection of constructs. Among these constructs, three general classes can be distinguished: algorithms, objects and types, which have the distinction that they can be denoted by names. The program text consists of such entities, glued together by control structures, data structures and further composition mechanisms such as expressions and functional application.