parameter (pair (list string) (list string));
storage (option bool);
code {CAR; DUP; CAR; DIP{CDR}; EMPTY_SET string; SWAP;
      ITER {PAIR; DUP; CAR; DIP{CDR}; PUSH bool True; SWAP; UPDATE};
      PUSH bool True; SWAP; PAIR; SWAP;
      ITER {PAIR; DUP; DUP; CAR; DIP{CDAR; DIP{CDDR}; DUP}; MEM; DIP{SWAP}; AND; SWAP; PAIR};
      CDR; SOME; NIL operation; PAIR};