thus Sum (digits (312,10)) = (Sum <%2,1%>) + (Sum <%3%>) by Th1050, AFINSQ_2:55
.= (2 + 1) + (Sum <%3%>) by AFINSQ_2:54
.= 3 + 3 by AFINSQ_2:53
.= 6 ; :: thesis: verum