thus Sum (digits (1403,10)) = (Sum <%3,0,4%>) + (Sum <%1%>) by Th1023, AFINSQ_2:55
.= ((Sum <%3,0%>) + (Sum <%4%>)) + (Sum <%1%>) by AFINSQ_2:55
.= ((3 + 0) + (Sum <%4%>)) + (Sum <%1%>) by AFINSQ_2:54
.= ((3 + 0) + 4) + (Sum <%1%>) by AFINSQ_2:53
.= 7 + 1 by AFINSQ_2:53
.= 8 ; :: thesis: verum