# A dash of infinity

## Recursion

So far, our models of coffee machines only modelled a single transaction–after dispensing a single cup of coffee, the machine terminated. In many situations in real life, however, we wish to model systems that repeatedly perform the same procedures.

The recipe for this is simple. We give a process a name, say \(P\), and then say that \(P\) may exhibit some finite behaviour, after which it will once more behave like \(P\). Let us create an optimistic model of a coffee machine, that lets us operate the machine forever.

```
act coin, good, bad;
proc P = coin .
(bad . P +
coin . good . P);
init P;
``` |

In the specification, we see that the `proc`

operator accepts equations that
associate processes with process names. In this case, the process `P`

is
defined. By using it in the right-hand side of its own definition, we introduced
infinitely repeating behaviour. To illustrate this, we could *unfold* this
repetition once and obtain a bisimilar system, as shown in
the figure of the unfolded ever-lasting coffee machine.

```
act coin, good, bad;
proc P = coin .
(bad . P +
coin . good . P);
init coin .
(bad . P +
coin . good . P);
``` |

Note that the definition of bisimilarity does not have to be changed to deal with recursive systems; the co-inductive definition guarantees that the future behaviour stays the same.

Exercise

Show that the statespaces from figures the ever-lasting coffee machine and the unfolded ever-lasting coffee machine are bisimilar.

## Regular HML

We saw that recursion does not require the definition of bisimilarity to be changed. Similarly, HML is still adequate to distinguish recursive systems: if two finite state systems are not bisimilar, then there is a finite HML expression that distinguishes the two. However, when we are dealing with infinitary systems, we often want to express properties of a system that say that it will always keep doing something, or that it will eventually do something. Such properties cannot be expressed by HML expressions of finite length.

To remedy this shortcoming, HML can be extended to allow regular expressions over action formulas inside the \(\mccan{\cdot}\) and \(\mcall{\cdot}\) modalities. In particular, the Kleene star is a powerful operator that effectively abbreviates certain HML formulas of infinite size.

Definition (Regular HML)

A *regular HML* formula interpreted over an LTS with action labels \(\act\)
adheres to the following syntax in pseudo-BNF, where \(A\) is an action formula
over \(\act\).

The usual abbreviations are allowed, as well as writing \(\alpha^+\) for \(\alpha\cdot\alpha^*\). We will informally give the semantics by rewriting regular HML formulas to infinitary HML formulas:

Exercise

Rewrite the regular HML formula \([a+b]\false\) to a pure HML formula.

## The modal µ-calculus

Although regular HML is a powerful tool to specify properties over infinite
systems, it is still not expressive enough to formulate so-called
fairness properties*. These are properties that say things like *if the system is offered
the possibility to perform an action infinitely often, then it will eventually
perform this action*.

Another way of extending HML to deal with infinite behaviour is to add a
*least fixpoint operator*. This extension is called the *modal µ-calculus*, named
after the least fixpoint operator \(\mu\). The µ-calculus (we often leave out the
*modal*, as no confusion can arise) is famous for its expressivity, and infamous
for its unintelligability. We will therefore first give the definition and the
formal semantics, and then elaborate more informally on its use.

Definition (µ-calculus)

A *µ-calculus* formula interpreted over an LTS with action labels \(\act\)
adheres to the following syntax in pseudo-BNF, where \(A\) is an action formula
over \(\act\), and \(X\) is chosen from some set of variable names \(\mathcal{X}\).

We allow the same abbreviations as for HML, and we add the *greatest fixpoint
operator*, which is the dual of the least fixpoint operator:

In the above, we use \(\varphi[\neg X/X]\) to denote \(\varphi\) with all occurrences of \(X\) replaced by \(\neg X\).

For technical reasons, we impose an important restriction on the syntax of
µ-calculus formulas: only formulas in which every use of a fixpoint variable
from \(\mathcal{X}\) is preceded by an even number of negations are allowed. The
formula is then in *positive normal form*, allowing us to give it a proper
semantics [1].

A µ-calculus formula \(\varphi\) is interpreted over an LTS
\(T = \langle S, \act, \rightarrow, i, f \rangle\). To accomodate the fixpoint
variables, we also need a *predicate environment} `rho: mathcal{X} to 2^S`,
which maps variable names to their semantics (*i.e.*, sets of states from
\(T\)). We use \(\rho[X\mapsto V]\) to denote the environment that maps \(X\) to the
set \(V\), and that maps all other variable names in the same way \(\rho\) does.

The semantics of a formula is now given as the set of states \(\sem{\varphi}_T^\rho \subseteq S\), defined as follows.

We say that \(T\) *satisfies* \(\varphi\), denoted \(T \models \varphi\), if and only
if \(i \in \sem{\varphi}_T^\rho\) for any \(\rho\).

### Using the µ-calculus

To understand how the µ-calculus can be used to express properties of systems, it is instructive to see that regular HML can be encoded into the modal µ-calculus by using the following equalities.

Intuitively, the least fixpoint operator \(\mu\) corresponds to an eventuality, where the greatest fixpoint operator says something about properties that continue to hold forever.

We can read \(\mu X\,.\, \varphi \lor \mccan{\alpha}X\) as \(X\) is the smallest set of states such that a state is in \(X\) if and only if \(\varphi\) holds in that state, or there is an \(\alpha\)-successor that is in \(X\)’. Conversely, \(\nu X\,.\, \varphi \land \mcall{\alpha}X\) is the largest set of states such that a state is in \(X\) if and only if \(\varphi\) holds in that state and all of its \(\alpha\)-successors are in \(X\).

A good way to learn how the µ-calculus works is by understanding how the
semantics of a formula can be computed. To do so, we use *approximations*. For
each fixpoint we encounter, we start with an initial approximation, and then
keep refining the approximation until the last two refinements are the same. The
current approximation is then a *fixpoint* of the formula, which is what we
were after. The first approximation \(\hat{X}^0\) for a fixpoint \(\mu X \,.\,
\varphi\) is given by \(\varphi[\false / X]\). For a greatest fixpoint \(\nu X \,.\,
\varphi\), it is given by \(\varphi[\true / X]\). In other words, for a least
fixpoint operator the initial approximation represents the empty set of states,
and for a greatest fixpoint operator we initially assume the formula holds for
all states. Each next approximation \(\hat{X}^{i+1}\) is given by
\(\varphi[\hat{X}^i / X]\). If \(\hat{X}^{i+1} = \hat{X}^i\), then we have reached
our fixpoint.

Example

Consider the following formula, which states that a coffee machine will always give coffee after a finite number of steps.

Note that this formula cannot be expressed using regular expressions. To see how the formula works, consider \(\hat{X}^0 =\mccan{\true}\true \land \mcall{\overline{\a{coffee}}}\false\). The first conjunct of this first approximation says that an action can be performed, and the second conjunct says that any action that can be performed must be a \(\a{coffee}\) action. The first approximation hence represents the set of states that can–and can only–do \(\a{coffee}\) actions.

The next approximation is \(\hat{X}^1 = \mccan{\true}\true \land
\mcall{\overline{\a{coffee}}} \hat{X}^0\). The first conjunct again selects
all states that may perform an action, and the second conjunct now selects
those that can additionally do only \(\a{coffee}\) actions, *or* that can do
another action and then always end up in the set of states where \(\hat{X}^0\)
holds. Continuing this reasoning, it is easy to see that \(\hat{X}^i\)
represents the set of states that must reach a state that must do a
\(\a{coffee}\) action in \(i\) or less steps. Hence, when we find a fixpoint, this
fixpoint represents those states that must eventually reach a state from
which a \(\a{coffee}\) action must be performed.

More complicated properties can be expressed by nesting fixpoint operators.

Exercise

What does the formula \(\nu X \,.\, \varphi \land \mccan{a}X\) express? Can it be expressed in regular HML?

## Data

Recursion is one way to introduce infinity in system models. It neatly
enables us to model systems that continuously interact with their
environment. The infinity obtained by recursion is an infinity in the
*depth* of the system. There is another form of infinity that we have
not yet explored: infinity in the width of the system. This type of
infinity can be obtained by combining processes and data.

We first illustrate the idea of combining processes and data with a simple
example. Let us reconsider the `coin`

action of the coffee machine.
Rather than assuming that there is only one flavour
of coins, there are in fact various types of coins: 2, 5 and 10 cents;
these values can be thought of as elements of the structured
sort `Val`

, defined as:

```
sort Val = struct c2 | c5 | c10;
```

The action `coin`

can be thought of as inserting
a particular type of coin, the value of which is dictated by a parameter
of the action. Thus, `coin(c2)`

represents the insertion of a
2 cent coin, whereas `coin(c10)`

represents the insertion of a
10 cents coin. Below, we have a state that accepts all possible
coins, with on the right the required mCRL2 notation.

```
sort Val = struct c2 | c5 | c10;
act coin: Val;
init sum v: Val . coin(v);
``` |

The statement `sum v: Val . coin(v)`

actually binds a local variable
`v`

of sort `Val`

, and, for every of its possible values,
specifies a `coin`

action with that value as a parameter.
An alternative description of the same process is

```
init coin(c2) + coin(c5) + coin(c10);
```

This suggests that the summation is like the plus.

As soon as the sort that is used in combination with the `sum`

operator has infinitely many basic elements, the branching degree of
a state may become infinite, as illustrated by figure Transition system with an infinite number of transitions..
Since each mCRL2 expression is finite, we can no longer give an
equivalent expression using only the plus operator.

```
act num: Nat;
init sum v: Nat . num(2 * v);
``` |

The sum operator is quite powerful, especially when combined with the *if-then*
construct `b -> p`

and the *if-then-else* construct `b -> p <> q`

, which
behaves as process `p`

if `b`

evaluates to `true`

, and, in case of the
if-then-else construct, as process `q`

otherwise. Using such constructs, and a
Boolean function `even`

, we can give an alternative description of the
infinite transition system above:

```
map even: Nat -> Bool;
var n: Nat;
eqn even(n) = n mod 2 == 0;
act num: Nat;
init sum v: Nat . even(v) -> num(v);
```

The Boolean condition `even(v)`

evaluates to `true`

or `false`

, dependent
on the value of `v`

. If, the expression `even(v)`

evaluates to `true`

,
action `num(v)`

is possible.

Exercise

Give a µ-calculus expression that states that this
process cannot execute actions `num`

with an odd natural
number as its parameter.

Data variables that are bound by the `sum`

operator can affect
the entire process that is within the scope of such operators. This way,
we are able to make the system behaviour data-dependent. Suppose, for instance,
that our coffee machine only accepts coins of 10 cents, and
rejects the 2 and 5 cent coins. The significant states modelling this behaviour,
including parts of the mCRL2 description,
are as follows:

```
sort Val = struct c2 | c5 | c10;
act coffee;
coin, rej: Val;
proc P =
sum v: Val . coin(v) . (
(v != c10) -> rej(v) . P
+ (v == c10) -> coffee . P
);
init P;
``` |

Data may also be used to parameterise recursion. A typical example of a process employing such mechanisms is an incrementer:

```
act num:Nat;
proc P(n:Nat) = num(n).P(n+1);
init P(0);
```

Or we could have written the picky coffee machine as follows:

```
proc P(v: Val) =
coin(v) . (
(v != c10) -> rej(v) . P
+ (v == c10) -> coffee . P
);
init sum v: Val . P(v);
```

It may be clear that most data-dependent processes describe transition systems that can no longer be visualised on a sheet of paper. However, the interaction between the data and process language is quite powerful.

Exercises

Is there a labelled transition system with a finite number of states that is bisimilar to the incrementer? If so, give this transition system and the witnessing bisimulation relation. If not, explain why such a transition system does not exist.

Consider the mCRL2 specification depicted below, defining a rather quirky coffee machine. List some odd things about the behaviour of this coffee machine and give an alternative specification that fixes these.

```
sort Val = struct c2 | c5 | c10;
map w: Val -> Nat;
eqn w(c2) = 2;
w(c5) = 5;
w(c10) = 10;
act insert_coin, return_coin: Val;
cancel, bad, good;
proc Loading(t: Int) =
sum v: Val .
insert_coin(v) . Loading(t + w(v))
+ (exists v: Val. t >= w(v)) -> cancel . Flushing(t)
+ (t >= 10) -> bad . Loading(t - 10)
+ (t >= 20) -> good . Loading(t - 20);
Flushing(t: Int) =
sum v: Val . sum t': Nat .
(t == t' + w(v)) -> return_coin(v) . Flushing(t')
+ (forall v: Val . w(v) > t) -> Loading(t);
init Loading(0);
``` |

## The first-order µ-calculus

With the introduction of data-dependent behaviour and, in particular, with the sum operator, we have moved beyond labelled transition systems that are finitely branching. As you may have found out in this exercise, the logics defined in the previous sections are no longer adequate to reason about the systems we can now describe. This is due to the fact that our grammar does not permit us to construct infinite sized formulae. We mend this by introducing data in the µ-calculus. This is done gently: first, we extend Hennessy-Milner logic to deal with the infinite branching.

Consider the action formulae of Hennessy-Milner logic. It allows one
to describe a set of actions. The actions in our LTSs are of a particular
shape: they start with an action name `a`

, taken from a finite
domain of action names, and they carry parameters of a particular sort,
which can possibly be an infinite sized sort. What we shall do is
extend the Hennessy-Milner action formulae with the facilities to
reason about the possible values these expressions can have. This is
most naturally done using quantifiers.

Definition (Action formulae)

An action formula over a set of action names \(\act\) is an expression that adheres to the following syntax in pseudo-BNF, where \(a \in \act\), \(d\) is a data variable, \(b\) is a Boolean expression, \(e\) is a data expression and \(D\) is a data sort.

The following abbreviations may also be used:

Since our action formulae may now refer to *data variables*, the meaning
of a formula necessarily depends on the value this variable has. The
assignment of values to variables is recorded in a mapping \(\varepsilon\).
An action formula \(A\) over \(\act\) is associated with a set
\(\sem{A}{\varepsilon} \subseteq \{a(v) ~|~ a \in \act \}\)
in the following manner.

Remark

Note that the function \(\varepsilon\) is used to assign concrete values to variables and extends easily to expressions. Consider, for instance, the Boolean expression \(b \wedge c\), where \(b\) and \(c\) are Boolean variables. Suppose that function \(\varepsilon\) states that \(\varepsilon(b) = \varepsilon(c) = \true\). Then \(\sem{b \wedge c}{\varepsilon} = \varepsilon(b \wedge c) = \varepsilon(b) \wedge \varepsilon(c) = \true \wedge \true = \true\).

The extension of our action formulae with data is sufficiently powerful to reason about the infinite branching introduced by the sum operator over infinite data sorts. However, it still does not permit us to reason about data-dependent behaviour. Consider, for instance, the LTS described by the following process:

```
act num: Nat;
proc P(n: Nat) = sum m: Nat . (m < n) -> num(m) . P(m);
init sum m: Nat . P(m);
``` |

Each `num(v)`

action leads to a state with branching degree \(v\), in which the
only actions `num(w)`

possible are those with `w < v`

. Using Hennessy-Milner
logic combined with our new action formulae fails to allow us to express that
from the initial state, no action `num(v)`

can be followed by an action
`num(v')`

for which `v <= v'`

. We can mend this by also extending the
grammar for Hennessy-Milner logic.

Definition (First-order HML)

A *First-order Hennessy-Milner logic* formula interpreted over an LTS with
action labels \(\act\) adheres to the following syntax in pseudo-BNF, where \(A\)
is an action formula over \(\act\), \(b\) is a Boolean expression, \(d\) is a data
variable and \(D\) is a data sort.

The following common abbreviations are allowed:

An HML formula \(\varphi\) is interpreted over an LTS \(T = \langle S, \act, \rightarrow, i, f \rangle\), and in the context of a data variable valuation function \(\varepsilon\). Its semantics is given as the set of states \(\sem{\varphi}_T^\varepsilon \subseteq S\) of the LTS in which the formula holds. It is defined as follows.

We say that \(T\) *satisfies* \(\varphi\), denoted \(T \models \varphi\), if and only
if for all \(\varepsilon\), \(i \in \sem{\varphi}_T^\varepsilon\).

Example

The property that from the initial state the \(\a{num}(v)\) action cannot be followed by a \(\a{num}(v')\) action with \(v' \geq v\) can now be written in a number of ways, one of them being \(\forall_{v,v'\oftype\nat} \mcall{\a{num}(v)}\mcall{\a{num}(v')} v' < v\).

The regular first-order Hennessy-Milner logic extends the first-order
Hennessy-Milner logic in the same way as regular Hennessy Milner logic
extends Hennessy-Milner logic. This allows us, for instance, to express that along all
paths of the LTS described by this transition system, the parameters
of the action `num`

are decreasing:

In a similar vein, the µ-calculus can be extended with first-order constructs,
allowing for *parameterised recursion*. This allows one to pass on
data values and use these to record events that have been observed in
the past.

Footnotes