thus Sum (digits (625,10)) = (Sum <%5,2%>) + (Sum <%6%>) by Th1097, AFINSQ_2:55
.= (5 + 2) + (Sum <%6%>) by AFINSQ_2:54
.= 7 + 6 by AFINSQ_2:53
.= 13 ; :: thesis: verum