thus Sum (digits (1680,10)) = (Sum <%0,8,6%>) + (Sum <%1%>) by Th496, AFINSQ_2:55
.= ((Sum <%0,8%>) + (Sum <%6%>)) + (Sum <%1%>) by AFINSQ_2:55
.= ((0 + 8) + (Sum <%6%>)) + (Sum <%1%>) by AFINSQ_2:54
.= ((0 + 8) + 6) + (Sum <%1%>) by AFINSQ_2:53
.= 14 + 1 by AFINSQ_2:53
.= 15 ; :: thesis: verum