thus Sum (digits (525,10)) = (Sum <%5,2%>) + (Sum <%5%>) by Th1095, AFINSQ_2:55
.= (5 + 2) + (Sum <%5%>) by AFINSQ_2:54
.= 7 + 5 by AFINSQ_2:53
.= 12 ; :: thesis: verum