thus Sum (digits (125,10)) = (Sum <%5,2%>) + (Sum <%1%>) by Th1077, AFINSQ_2:55
.= (5 + 2) + (Sum <%1%>) by AFINSQ_2:54
.= 7 + 1 by AFINSQ_2:53
.= 8 ; :: thesis: verum