----------------- ->
                                                a->SKIP -a-> SKIP
------------------- ->                          ----------------------------------- ;1
a -> SKIP -a-> SKIP                             (a->SKIP);a->STOP -a-> SKIP;a->STOP
------------------------------------------- ;1  -------------------------------------------------- []3
((a -> SKIP);(b->STOP)) -a-> SKIP;(b->STOP)     ((a->SKIP);a->STOP) [] b->STOP -a-> SKIP;a->STOP
--------------------------------------------------------------------------------------------------------- [||]
    ((a -> SKIP);(b->STOP))
  [|{|a|}|]
    (((a->SKIP);a->STOP) [] b->STOP)
-a->
    (SKIP;(b->STOP))
  [|{|a|}|]
    (SKIP;a->STOP)