thus Sum (digits (216,10)) = (Sum <%6,1%>) + (Sum <%2%>) by Th1048, AFINSQ_2:55
.= (6 + 1) + (Sum <%2%>) by AFINSQ_2:54
.= 7 + 2 by AFINSQ_2:53
.= 9 ; :: thesis: verum