thus Sum (digits (3950,10)) = (Sum <%0,5,9%>) + (Sum <%3%>) by Th1293, AFINSQ_2:55
.= ((Sum <%0,5%>) + (Sum <%9%>)) + (Sum <%3%>) by AFINSQ_2:55
.= ((0 + 5) + (Sum <%9%>)) + (Sum <%3%>) by AFINSQ_2:54
.= ((0 + 5) + 9) + (Sum <%3%>) by AFINSQ_2:53
.= 14 + 3 by AFINSQ_2:53
.= 17 ; :: thesis: verum