thus Sum (digits (744,10)) = (Sum <%4,4%>) + (Sum <%7%>) by Th1064, AFINSQ_2:55
.= (4 + 4) + (Sum <%7%>) by AFINSQ_2:54
.= 8 + 7 by AFINSQ_2:53
.= 15 ; :: thesis: verum