thus Sum (digits (825,10)) = (Sum <%5,2%>) + (Sum <%8%>) by Th1109, AFINSQ_2:55
.= (5 + 2) + (Sum <%8%>) by AFINSQ_2:54
.= 7 + 8 by AFINSQ_2:53
.= 15 ; :: thesis: verum