code { LOOP_LEFT { DUP; CAR; DIP{CDR};
                   IF_CONS { SWAP; DIP{CONS}; PAIR; LEFT (list string) }
                           { RIGHT (pair (list string) (list string)) } } } ;
input { Stack_elt (or (pair (list string) (list string)) (list string)) (Left (Pair { } { }))  } ;
output { Stack_elt (list string) { } }