thus Sum (digits (3960,10)) = (Sum <%0,6,9%>) + (Sum <%3%>) by Th724, AFINSQ_2:55
.= ((Sum <%0,6%>) + (Sum <%9%>)) + (Sum <%3%>) by AFINSQ_2:55
.= ((0 + 6) + (Sum <%9%>)) + (Sum <%3%>) by AFINSQ_2:54
.= ((0 + 6) + 9) + (Sum <%3%>) by AFINSQ_2:53
.= 15 + 3 by AFINSQ_2:53
.= 18 ; :: thesis: verum