thus Sum (digits (325,10)) = (Sum <%5,2%>) + (Sum <%3%>) by Th1087, AFINSQ_2:55
.= (5 + 2) + (Sum <%3%>) by AFINSQ_2:54
.= 7 + 3 by AFINSQ_2:53
.= 10 ; :: thesis: verum