 Compositionality (Szabó 2013) is a desirable feature of a semantical analysis of a programming language, because it allows one to calculate the meaning of a program as a function of the meanings of its constituents. Actually, in Milner’s account (see especially 1975: sec. 1, 4), compositionality applies even more generally to computing agents assembled from smaller ones by means of appropriate composition operations. These agents may include, beside programs, hardware systems like a computer composed of a memory, composed in turn of two memory registers, and a processing unit, where all components are computing agents. This allows one to include in one framework systems composed of hardware, of software and even of both. Now, the syntactic rules that define inductively the various categories of phrases of a programming language allow us to regard \({L}\) as an algebra of program phrases, whose signature is determined by these rules. One account of compositionality that is especially suitable to the present setting (Szabó 2013: sec. 2) identifies a compositional interpretation of programs with a homomorphism from this algebra to the domain of denotations associating with every operation of the algebra of programs a corresponding semantical operation on denotations.