thus Sum (digits (768,10)) = (Sum <%8,6%>) + (Sum <%7%>) by Th1066, AFINSQ_2:55
.= (8 + 6) + (Sum <%7%>) by AFINSQ_2:54
.= 14 + 7 by AFINSQ_2:53
.= 21 ; :: thesis: verum