thus Sum (digits (1280,10)) = (Sum <%0,8,2%>) + (Sum <%1%>) by Th456, AFINSQ_2:55
.= ((Sum <%0,8%>) + (Sum <%2%>)) + (Sum <%1%>) by AFINSQ_2:55
.= ((0 + 8) + (Sum <%2%>)) + (Sum <%1%>) by AFINSQ_2:54
.= ((0 + 8) + 2) + (Sum <%1%>) by AFINSQ_2:53
.= 10 + 1 by AFINSQ_2:53
.= 11 ; :: thesis: verum