Func FT( arr, n, m ) =
    ;{executes John H. Conway's FRACTRAN language for a program stored in [arr], an}
    ;{input integer stored in n, for a maximum of m steps}
    ;{To allow the program to run indefinitely, give it negative or noninteger m}
    exec:=1;                 {boolean to track whether the program needs to halt}
    len:=Cols[arr];          {length of the input program}
    while exec=1 and m<>0 do
        m:-;
        !!n;                 {output the memory}
        i:=1;                {index variable}
        exec:=0;
        while i<=len and exec=0 do
            nf:=n*arr[i];
            if Denom(nf) = 1 then
                n:=nf;       {did we find an instruction to execute?}
                exec:=1
            fi;
            i:+;
        od;
    od;
.;

;{Here is the program to run}
[arr]:=[( 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 )];

FT( [arr], 2, 20 );