thus Sum (digits (624,10)) = (Sum <%4,2%>) + (Sum <%6%>) by Th1056, AFINSQ_2:55
.= (4 + 2) + (Sum <%6%>) by AFINSQ_2:54
.= 6 + 6 by AFINSQ_2:53
.= 12 ; :: thesis: verum