thus Sum (digits (1350,10)) = (Sum <%0,5,3%>) + (Sum <%1%>) by Th1139, AFINSQ_2:55
.= ((Sum <%0,5%>) + (Sum <%3%>)) + (Sum <%1%>) by AFINSQ_2:55
.= ((0 + 5) + (Sum <%3%>)) + (Sum <%1%>) by AFINSQ_2:54
.= ((0 + 5) + 3) + (Sum <%1%>) by AFINSQ_2:53
.= 8 + 1 by AFINSQ_2:53
.= 9 ; :: thesis: verum