(fractran=
  np n fs A Z fi P p N L M
.   !arg:(?N,?n,?fs)                 {Number of iterations, start n, fractions}
  & :?P:?L                           {Initialise accumulators.}
  &   whl
    ' ( -1+!N:>0:?N                  {Stop when counted down to zero.}
      & !n !L:?L                     {Prepend all numbers to result list.}
      & (2\L!n:#?p&!P !p:?P|)        {If log2(n) is rational, append it to list of primes.}
      & !fs:? (/?fi&!n*!fi:~/:?n) ?  {This line does the following (See task description):
                                      "for the first fraction, fi, in the list for which
                                       nfi is an integer, replace n by nfi ;"}
      )
  & :?M
  & whl'(!L:%?n ?L&!n !M:?M)         {Invert list of numbers. (append to long list is
                                      very expensive. Better to prepend and finally invert.}
  & (!M,!P)                          {Return the two lists}
);



( clk$:?t0
& fractran$(430000, 2, 17/91 78/85 19/51 23/38 29/33 77/29 95/23 77/19 1/17 11/13 13/11 15/14 15/2 55/1)
  : (?numbers,?primes)
& lst$(numbers,"numbers.lst",NEW)
& put$("
FRACTRAN found these primes:"
  !primes
  "\nThe list of numbers is saved in numbers.txt
The biggest number in the list is"
  (   0:?max
    & !numbers:? (>%@!max:?max&~) ?
  | !max
  )
str$("\ntime: " flt$(clk$+-1*!t0,4) " sec\n")
, "FRACTRAN.OUT",NEW)
);