procedure swap(s)
static labels, trans, max
initial {
labels := "12"
trans := "21"
max := *labels
trans := swap(string(&cset))
labels := string(&cset)
max := *labels
}
if *s <= max then
return map(left(trans,*s),left(labels,*s),s)
else return swap(left(s,*s - max)) ||
map(trans,labels,right(s,max))
end