thus Sum (digits (225,10)) = (Sum <%5,2%>) + (Sum <%2%>) by Th1081, AFINSQ_2:55
.= (5 + 2) + (Sum <%2%>) by AFINSQ_2:54
.= 7 + 2 by AFINSQ_2:53
.= 9 ; :: thesis: verum