to_letter := proc(num)
    local cs, cn, sl, a, b, c, d, e, f, g, h, i, j, k,
          l, m, n, o, p, q, r, s, t, u, v, w, x, y, z,
          ltable, ans;
    ltable := table([0=a, 1=b, 2=c, 3=d, 4=e, 5=f, 6=g,
                     7=h, 8=i, 9=j, 10=k, 11=l, 12=m,
                     13=n, 14=o, 15=p, 16=q, 17=r, 18=s,
                     19=t, 20=u, 21=v, 22=w, 23=x, 24=y,
                     25=z]);
    cn := num;
    sl := floor(trunc(evalf(log10(cn)))/2) + 1:
    ans := ``;
    for i from 1 to sl do
        cn := cn/100;
        cs := ltable[frac(cn)*100];
        ans := cat(cs, ans);
        cn := trunc(cn);
    od:
    RETURN(ans);
end: