thus Sum (digits (675,10)) = (Sum <%5,7%>) + (Sum <%6%>) by Th1101, AFINSQ_2:55
.= (5 + 7) + (Sum <%6%>) by AFINSQ_2:54
.= 12 + 6 by AFINSQ_2:53
.= 18 ; :: thesis: verum