thus Sum (digits (816,10)) = (Sum <%6,1%>) + (Sum <%8%>) by Th1068, AFINSQ_2:55
.= (6 + 1) + (Sum <%8%>) by AFINSQ_2:54
.= 7 + 8 by AFINSQ_2:53
.= 15 ; :: thesis: verum