thus Sum (digits (1720,10)) = (Sum <%0,2,7%>) + (Sum <%1%>) by Th500, AFINSQ_2:55
.= ((Sum <%0,2%>) + (Sum <%7%>)) + (Sum <%1%>) by AFINSQ_2:55
.= ((0 + 2) + (Sum <%7%>)) + (Sum <%1%>) by AFINSQ_2:54
.= ((0 + 2) + 7) + (Sum <%1%>) by AFINSQ_2:53
.= 9 + 1 by AFINSQ_2:53
.= 10 ; :: thesis: verum