thus Sum (digits (504,10)) = (Sum <%4,0%>) + (Sum <%5%>) by Th1052, AFINSQ_2:55
.= (4 + 0) + (Sum <%5%>) by AFINSQ_2:54
.= 4 + 5 by AFINSQ_2:53
.= 9 ; :: thesis: verum