On improving efficiency of model checking through systematically ...

are transformed as follows:
b i prime := b i ,i=1...m
x k prime := x k ,k =1...p
b i := φ i ( b prime
⃗ ,⃗c, x prime ⃗ ,⃗y)
x i := ψ k ( b prime
⃗ ,⃗c, x prime ⃗ ,⃗y)
As such, the two assignments above are specified in XTG as follows:
b0_prime := b0;
b1_prime := b1;
b0 := b1_prime;
b1 := not b0_prime;
External environment. As described in Chapter 2, Nbac uses assertion
and input variables to model the external environment setting. At every
execution step (when a transition is taken), the assertion is consulted to obtain
new values for input variables. These values are set in a non-determistic
way provided that they satisfy the constraint stated by the assertion.
Hence, the external environment is actually specified implicitly in Nbac
in the sense that we just have to declare the variables and conditions. Nbac
takes care of other parts including generating relevant values in a nondeterministic
way and getting new values at the beginning of every execution
step. As a consequence, we have to model those processes in XTG explicitly.
Our solution to model the external environment involves two parts:
1. We introduce an additional graph namely input graph to generate values
for input variables. Values are generated in a non-deterministic
way and contrained by the assertion.
2. We set up a synchronous communication between the system graph and
the input graph in order to give input values to the system at every
execution step. This requires additional transitions and locations in
the system graph.
Modeling the communication between the system graph and the input
graph as a synchronous communication is a natural way due to the semantics
of the communication:
1. At every execution step, input values are consulted by the system
graph to compute new values for state variables and evaluate Boolean
expressions on locations and edges.
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