thus Sum (digits (576,10)) = (Sum <%6,7%>) + (Sum <%5%>) by Th1054, AFINSQ_2:55
.= (6 + 7) + (Sum <%5%>) by AFINSQ_2:54
.= 13 + 5 by AFINSQ_2:53
.= 18 ; :: thesis: verum