thus Sum (digits (1620,10)) = (Sum <%0,2,6%>) + (Sum <%1%>) by Th490, AFINSQ_2:55
.= ((Sum <%0,2%>) + (Sum <%6%>)) + (Sum <%1%>) by AFINSQ_2:55
.= ((0 + 2) + (Sum <%6%>)) + (Sum <%1%>) by AFINSQ_2:54
.= ((0 + 2) + 6) + (Sum <%1%>) by AFINSQ_2:53
.= 8 + 1 by AFINSQ_2:53
.= 9 ; :: thesis: verum