thus Sum (digits (3900,10)) = (Sum <%0,0,9%>) + (Sum <%3%>) by Th718, AFINSQ_2:55
.= ((Sum <%0,0%>) + (Sum <%9%>)) + (Sum <%3%>) by AFINSQ_2:55
.= ((0 + 0) + (Sum <%9%>)) + (Sum <%3%>) by AFINSQ_2:54
.= ((0 + 0) + 9) + (Sum <%3%>) by AFINSQ_2:53
.= 9 + 3 by AFINSQ_2:53
.= 12 ; :: thesis: verum