thus Sum (digits (1716,10)) = (Sum <%6,1,7%>) + (Sum <%1%>) by Th862, AFINSQ_2:55
.= ((Sum <%6,1%>) + (Sum <%7%>)) + (Sum <%1%>) by AFINSQ_2:55
.= ((6 + 1) + (Sum <%7%>)) + (Sum <%1%>) by AFINSQ_2:54
.= ((6 + 1) + 7) + (Sum <%1%>) by AFINSQ_2:53
.= 14 + 1 by AFINSQ_2:53
.= 15 ; :: thesis: verum