@@ -61,60 +61,74 @@ The function space $(-) \nfn (-)$ is a locally non-expansive bifunctor.

Note that the composition of non-expansive (bi)functors is non-expansive, and the composition of a non-expansive and a contractive (bi)functor is contractive.

RAs are closely related to \emph{Partial Commutative Monoids} (PCMs), with two key differences:

\begin{enumerate}

\item The composition operation on RAs is total (as opposed to the partial composition operation of a PCM), but there is a specific subset of \emph{valid} elements that is compatible with the operation (\ruleref{ra-valid-op}).

\item Instead of a single unit that is an identity to every element, there is a function $\mcore{-}$ assigning to every element $\melt$ its \emph{(duplicable) core}$\mcore\melt$, as demanded by \ruleref{ra-core-id}. \\

We further demand that $\mcore{-}$ is idempotent (\ruleref{ra-core-idem}) and monotone (\ruleref{ra-core-mono}) with respect to the usual \emph{extension order}, which is defined similar to PCMs (\ruleref{ra-incl}).

\item The composition operation on RAs is total (as opposed to the partial composition operation of a PCM), but there is a specific subset $\mval$ of \emph{valid} elements that is compatible with the composition operation (\ruleref{ra-valid-op}).

This idea of a core is closely related to the concept of \emph{multi-unit separation algebras}~\cite{Dockins+:aplas09}, with the key difference that the core is a \emph{function} defining a \emph{canonical} ``unit'' $\mcore\melt$ for every element~$\melt$.

This take on partiality is necessary when defining the structure of \emph{higher-order} ghost state, CMRAs, in the next subsection.

\item Instead of a single unit that is an identity to every element, we allow

for an arbitrary number of units, via a function $\mcore{{-}}$ assigning to an element $\melt$ its \emph{(duplicable) core}$\mcore\melt$, as demanded by \ruleref{ra-core-id}.

We further demand that $\mcore{{-}}$ is idempotent (\ruleref{ra-core-idem}) and monotone (\ruleref{ra-core-mono}) with respect to the \emph{extension order}, defined similarly to that for PCMs (\ruleref{ra-incl}).

Notice that the domain of the core is $\maybe\monoid$, a set that adds a dummy element $\dummymelt$ to $\monoid$.

% (This corresponds to the option type.)

Thus, the core can be \emph{partial}: not all elements need to have a unit.

We use the metavariable $\maybe\melt$ to indicate elements of $\maybe\monoid$.

We also lift the composition $(\mtimes)$ to $\maybe\monoid$.

Partial cores help us to build interesting composite RAs from smaller primitives.

Notice also that the core of an RA is a strict generalization of the unit that any PCM must provide, since $\mcore{{-}}$ can always be picked as a constant function.

\end{enumerate}

\begin{defn}

It is possible to do a \emph{frame-preserving update} from $\melt\in\monoid$ to $\meltsB\subseteq\monoid$, written $\melt\mupd\meltsB$, if

We further define $\melt\mupd\meltB\eqdef\melt\mupd\set\meltB$.

\end{defn}

The assertion $\melt\mupd\meltsB$ says that every element $\melt_\f$ compatible with $\melt$ (we also call such elements \emph{frames}), must also be compatible with some $\meltB\in\meltsB$.

The assertion $\melt\mupd\meltsB$ says that every element $\maybe{\melt_\f}$ compatible with $\melt$ (we also call such elements \emph{frames}), must also be compatible with some $\meltB\in\meltsB$.

Notice that $\maybe{\melt_\f}$ could be $\dummymelt$, so the frame-preserving update can also be applied to elements that have \emph{no} frame.

Intuitively, this means that whatever assumptions the rest of the program is making about the state of $\gname$, if these assumptions are compatible with $\melt$, then updating to $\meltB$ will not invalidate any of these assumptions.

Since Iris ensures that the global ghost state is valid, this means that we can soundly update the ghost state from $\melt$ to a non-deterministically picked $\meltB\in\meltsB$.

\subsection{CMRA}

\begin{defn}

A \emph{CMRA} is a tuple $(\monoid : \COFEs, (\mval_n \subseteq\monoid)_{n \in\mathbb{N}},\mcore{-}: \monoid\nfn\monoid, (\mtimes) : \monoid\times\monoid\nfn\monoid)$ satisfying

A \emph{CMRA} is a tuple $(\monoid : \COFEs, (\mval_n \subseteq\monoid)_{n \in\mathbb{N}},\\\mcore{{-}}: \monoid\nfn\maybe\monoid, (\mtimes) : \monoid\times\monoid\nfn\monoid)$ satisfying:

\begin{align*}

\All n, \melt, \meltB.&\melt\nequiv{n}\meltB\land\melt\in\mval_n \Ra\meltB\in\mval_n \tagH{cmra-valid-ne}\\

\All n, m.& n \geq m \RaV_n \subseteq V_m \tagH{cmra-valid-mono}\\

\All n, m.& n \geq m \Ra\mval_n \subseteq\mval_m \tagH{cmra-valid-mono}\\

@@ -143,12 +157,10 @@ The purpose of this axiom is to compute $\melt_1$, $\melt_2$ completing the foll

\end{tikzpicture}\end{center}

where the $n$-equivalence at the bottom is meant to apply to the pairs of elements, \ie we demand $\melt_1\nequiv{n}\meltB_1$ and $\melt_2\nequiv{n}\meltB_2$.

In other words, extension carries the decomposition of $\meltB$ into $\meltB_1$ and $\meltB_2$ over the $n$-equivalence of $\melt$ and $\meltB$, and yields a corresponding decomposition of $\melt$ into $\melt_1$ and $\melt_2$.

This operation is needed to prove that $\later$ commutes with existential quantification and separating conjunction:

This operation is needed to prove that $\later$ commutes with separating conjunction: