Action semantics is a semantic description framework with very good pragmatic properties but until now a rather weak theory for reasoning about programs. A strong action theory would have a great practical potential, as it would facilitate reasoning about the large class of programming languages that can be described in action semantics.

Syntax Tree as input. The algorithm traverses the tree in a Modular SOS fashion: the traversal is directed by combinators, until a subtree can be collapsed. The tree thus decreases in depth in a bottom up fashion, while traversing it top down for each step. The main di#erence between acr and evalan is that acr employs term rewriting only to reduce combined actions. All primitive actions (including data operations and predicates) are hardcoded in C. Especially for interacting actions the performance gain is expected to be more than marginal since no matching is involved in rotating the schedule (and this is likely to occur very often).

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