thus Sum (digits (696,10)) = (Sum <%6,9%>) + (Sum <%6%>) by Th1062, AFINSQ_2:55
.= (6 + 9) + (Sum <%6%>) by AFINSQ_2:54
.= 15 + 6 by AFINSQ_2:53
.= 21 ; :: thesis: verum