thus Sum (digits (175,10)) = (Sum <%5,7%>) + (Sum <%1%>) by Th1079, AFINSQ_2:55
.= (5 + 7) + (Sum <%1%>) by AFINSQ_2:54
.= 12 + 1 by AFINSQ_2:53
.= 13 ; :: thesis: verum