thus Sum (digits (450,10)) = (Sum <%0,5%>) + (Sum <%4%>) by Th1093, AFINSQ_2:55
.= (0 + 5) + (Sum <%4%>) by AFINSQ_2:54
.= 5 + 4 by AFINSQ_2:53
.= 9 ; :: thesis: verum