thus Sum (digits (672,10)) = (Sum <%2,7%>) + (Sum <%6%>) by Th1060, AFINSQ_2:55
.= (2 + 7) + (Sum <%6%>) by AFINSQ_2:54
.= 9 + 6 by AFINSQ_2:53
.= 15 ; :: thesis: verum