program recaman;
    a := {[0,0]};

    loop for i in [1..14] do
        extend(a);
    end loop;

    print("First 15:", [a(n) : n in [0..14]]);

    loop
        doing n := extend(a);
        until #(rept:=[[r,i] : r = a(i) | r=n]) > 1
        do pass;
    end loop;

    print("First repetition:", n, "at", {x:x in rept}{n});

    proc extend(rw a);
        n := max/ domain a;
        t := a(n) - n-1;
        if t<0 or t in range a then
            t := a(n) + n+1;
        end if;
        return a(n+1) := t;
    end proc;
end program;