Calling conventions are essential for the operation of modular code. While they are necessary for understanding low-level computation, it is often a difficult topic for students. Writing assembly code itself is difficult without an extra set of constraints placed on top. In this paper, we detail a double act approach to not only making calling conventions accessible, but obvious and intuitive.

Abstract In the theory of coalgebras, distributive laws give a general perspective on determinisation and other automata constructions. This perspective has recently been extended to include so-called weak distributive laws, covering several constructions on state-based systems that are not captured by regular distributive laws, such as the construction of a belief-state transformer from a probabilistic automaton, and ultrafilter extensions of Kripke frames. In this paper we first observe that weak distributive laws give rise to the more general notion of what we call an invertible step: a pair of natural transformations that allows to move coalgebras along an adjunction. Our main result is that part of the construction induced by an invertible step preserves and reflects bisimilarity. This covers results that have previously been shown by hand for the instances of ultrafilter extensions and belief-state transformers.