thus Sum (digits (864,10)) = (Sum <%4,6%>) + (Sum <%8%>) by Th1070, AFINSQ_2:55
.= (4 + 6) + (Sum <%8%>) by AFINSQ_2:54
.= 10 + 8 by AFINSQ_2:53
.= 18 ; :: thesis: verum