thus Sum (digits (4050,10)) = (Sum <%0,5,0%>) + (Sum <%4%>) by Th1301, AFINSQ_2:55
.= ((Sum <%0,5%>) + (Sum <%0%>)) + (Sum <%4%>) by AFINSQ_2:55
.= ((0 + 5) + (Sum <%0%>)) + (Sum <%4%>) by AFINSQ_2:54
.= ((0 + 5) + 0) + (Sum <%4%>) by AFINSQ_2:53
.= 5 + 4 by AFINSQ_2:53
.= 9 ; :: thesis: verum