thus Sum (digits (850,10)) = (Sum <%0,5%>) + (Sum <%8%>) by Th1111, AFINSQ_2:55
.= (0 + 5) + (Sum <%8%>) by AFINSQ_2:54
.= 5 + 8 by AFINSQ_2:53
.= 13 ; :: thesis: verum