Algol, observational equivalence
Programming Languages and Compilers
A simple Idealized Algol is considered, based on Reynolds's "essence of Algol." It is shown that observational equivalence in this language conservatively extends observational equivalence in its assignment-free functional sublanguage.
O'Hearn, Peter W., "Note on Algol and Conservatively Extending Functional Programming" (1995). L.C. Smith College of Engineering and Computer Science - Former Departments, Centers, Institutes and Projects. Paper 32.