(RULES
  twoto 0 -> p p s s s p p p s s s 0 p p s s ,
  twoto s -> p p s s p p p s s s twice p p s s p p p s s s twoto p s p s ,

  twice 0 -> p s p s 0 s p s s s s p s ,
  twice s -> s p p p p s s s s s twice p s p s p s p s ,

  p p s -> p ,
  p s -> ,
  p 0 -> 0 s s s s p s ,
  0 -> 
)