A computation of an ASM is a finite or infinite sequence
s0; s1; : : : ; sn; : : : of states of the machine, where s0 is an
initial state and each sn+1 is obtained from sn by simultaneously
firing all the transition rules which are enabled in sn.
The (unique) main rule is a transition rule and represents the
starting point of the computation. An ASM can have more
than one initial state. It is possible to specify state invariants.