thus Sum (digits (3150,10)) = (Sum <%0,5,1%>) + (Sum <%3%>) by Th1245, AFINSQ_2:55
.= ((Sum <%0,5%>) + (Sum <%1%>)) + (Sum <%3%>) by AFINSQ_2:55
.= ((0 + 5) + (Sum <%1%>)) + (Sum <%3%>) by AFINSQ_2:54
.= ((0 + 5) + 1) + (Sum <%3%>) by AFINSQ_2:53
.= 6 + 3 by AFINSQ_2:53
.= 9 ; :: thesis: verum