Promela and Model Description
6) Case selection
  ex1:  if
:: (a != b) -> option1 // guard (a != b)
:: (a == b) -> option2 // guard (a==b)
             fi
  Only one case above will be executed according to the guard (first statement)
  a) What will happen if more that one guards are executable,  which one will
      run, option1 or option 2 ?  (nondeterministic)
  b) How if none of the guards is true?  (blocked)