 If \({\mathcal{M}}\) is compositional, then \({\simeq_{\mathcal{M}}}\) is a congruence over the algebra of programs, whose derived operations, those obtained by composition of operations of the signature, are called contexts. A context \(C\blbr\) represents a program phrase with a “hole” that can be filled by program phrases \(e\) of appropriate type to yield the program phrase \(C[e]\). By means of contexts we can characterize easily the compositionality of a semantic mapping: