Using game semantics we prove that program equivalence is undecidable in finitary Idealized Algol with active expressions as well as in its call-by-value counterpart. It is also shown that strategies corresponding to Idealized Algol terms of respectively second, third and higher orders define exactly regular, context-free and recursively enumerable languages.
['Andrzej S. Murawski']
On program equivalence in languages with ground-type references