Rewriting rules for event model decomposition

Rewriting rules for the Event-B assignments

As detailed in the modelling language, the Event-B assignments are formed of two parts:

• a left-hand side, which is a list of free identifiers.
• a right-hand side.

There are various kinds of assignments:

• The "becomes such that" assignment, $\bcmsuch$, is the most general (non-deterministic) assignment, where a predicate is given on the before and after values of assigned identifiers. The after values of the assigned identifiers are denoted by a primed identifier whose prefix is the assigned identifier.
• The "becomes equal to" assignment, $\bcmeq$ is the deterministic assignment, where an expression is given for each assigned identifier.
• The "becomes member of" assignment, $\bcmin$ is the set-based (non-deterministic) assignment, where a set expression is given for the assigned identifier.

Let $v$ and $w$ be variables, and $E$ and $F$ be expressions. In the following table, the left-hand assignments are equivalent (\defi) to the right-hand ones:

$v \bcmeq E$	$v \bcmsuch v' = E$
$v,w \bcmeq E,F$	$v \bcmeq E \pprod w \bcmeq F$
$v(E) \bcmeq F$	$v \bcmeq v \ovl \{E \mapsto F\}$
$v \bcmin E$	$v \bcmsuch v' \in E$

Thus, each Event-B assignment can be expressed in a "becomes such that" form.