thus Sum (digits (3420,10)) = (Sum <%0,2,4%>) + (Sum <%3%>) by Th670, AFINSQ_2:55
.= ((Sum <%0,2%>) + (Sum <%4%>)) + (Sum <%3%>) by AFINSQ_2:55
.= ((0 + 2) + (Sum <%4%>)) + (Sum <%3%>) by AFINSQ_2:54
.= ((0 + 2) + 4) + (Sum <%3%>) by AFINSQ_2:53
.= 6 + 3 by AFINSQ_2:53
.= 9 ; :: thesis: verum