thus Sum (digits (648,10)) = (Sum <%8,4%>) + (Sum <%6%>) by Th1058, AFINSQ_2:55
.= (8 + 4) + (Sum <%6%>) by AFINSQ_2:54
.= 12 + 6 by AFINSQ_2:53
.= 18 ; :: thesis: verum