The paper by Matthew Hennessy and Robin Milner introduces a novel approach to the semantics of nondeterministic and concurrent programs, focusing on observational equivalence. They define observational equivalence for programs or program parts, where two programs are equivalent if no observations can distinguish them. The behavior of a program is defined as its observation congruence class. The authors demonstrate that for a sequence of simple languages expressing finite behaviors, observation congruence can be axiomatized algebraically. By adding recursion and extending the language to allow communication between concurrent agents, the algebraic language becomes a calculus for writing and specifying concurrent programs and proving their properties. The paper presents a general framework for observational equivalence and provides detailed results for specific languages, including examples and proofs. The authors also explore the impact of unobservable actions on the observable behavior of programs and provide characterizations of observational congruence using axioms.The paper by Matthew Hennessy and Robin Milner introduces a novel approach to the semantics of nondeterministic and concurrent programs, focusing on observational equivalence. They define observational equivalence for programs or program parts, where two programs are equivalent if no observations can distinguish them. The behavior of a program is defined as its observation congruence class. The authors demonstrate that for a sequence of simple languages expressing finite behaviors, observation congruence can be axiomatized algebraically. By adding recursion and extending the language to allow communication between concurrent agents, the algebraic language becomes a calculus for writing and specifying concurrent programs and proving their properties. The paper presents a general framework for observational equivalence and provides detailed results for specific languages, including examples and proofs. The authors also explore the impact of unobservable actions on the observable behavior of programs and provide characterizations of observational congruence using axioms.