thus Sum (digits (925,10)) = (Sum <%5,2%>) + (Sum <%9%>) by Th1115, AFINSQ_2:55
.= (5 + 2) + (Sum <%9%>) by AFINSQ_2:54
.= 7 + 9 by AFINSQ_2:53
.= 16 ; :: thesis: verum