thus Sum (digits (875,10)) = (Sum <%5,7%>) + (Sum <%8%>) by Th1113, AFINSQ_2:55
.= (5 + 7) + (Sum <%8%>) by AFINSQ_2:54
.= 12 + 8 by AFINSQ_2:53
.= 20 ; :: thesis: verum