thus Sum (digits (4000,10)) = (Sum <%0,0,0%>) + (Sum <%4%>) by Th1297, AFINSQ_2:55
.= ((Sum <%0,0%>) + (Sum <%0%>)) + (Sum <%4%>) by AFINSQ_2:55
.= ((0 + 0) + (Sum <%0%>)) + (Sum <%4%>) by AFINSQ_2:54
.= ((0 + 0) + 0) + (Sum <%4%>) by AFINSQ_2:53
.= 4 by AFINSQ_2:53 ; :: thesis: verum