thus Sum (digits (3280,10)) = (Sum <%0,8,2%>) + (Sum <%3%>) by Th656, AFINSQ_2:55
.= ((Sum <%0,8%>) + (Sum <%2%>)) + (Sum <%3%>) by AFINSQ_2:55
.= ((0 + 8) + (Sum <%2%>)) + (Sum <%3%>) by AFINSQ_2:54
.= ((0 + 8) + 2) + (Sum <%3%>) by AFINSQ_2:53
.= 10 + 3 by AFINSQ_2:53
.= 13 ; :: thesis: verum