[ stack 0 ]              is seennumbers (     --> s   )

  [ bit seennumbers take
    | seennumbers put ]    is seen        (   n -->     )

  [ dup 0 < iff
      [ drop true ] done
    bit seennumbers share
    & 0 != ]                is seen?       (   n --> b   )

  [ 1+ bit 1 -
    seennumbers share
    over & = ]              is allseen?    (   n --> b   )

  [ stack [ ] ]             is repeats     (     --> s   )

  [ 1 seennumbers replace
    [] repeats replace
    ' [ 0 ] 1 ]             is startseq    (     --> [ n )

  [ over -1 peek
    over - dup seen? if
      [ over 2 * +
        dup seen? if
          [ repeats take
            over join
            repeats put ] ]
    dup seen
    swap dip join
    1+ ]                    is nextterm    ( [ n --> [ n )

  say "first 15 terms: "
  startseq
  14 times nextterm
  drop echo cr

  say "first duplicated term: "
  startseq
  [ repeats share [] = while
    nextterm
    again ]
  drop -1 peek echo cr

  say "terms needed to generate 0 to 1000: "
  startseq
  [ nextterm
    1000 allseen? until ]
  nip 1 - echo cr