Promela and Model Description
2) Atomic sequence
    Atomic sequence provides a monitor-like synchronization mechanism like
    what critical section does.
ex1:  byte state = 1;
          proctype A() {
atomic { (state==1) -> state = state+1 }
          }
          proctype B() {
atomic { (state==1) -> state = state-1 }
          }
         init { run A(); run B() }