thus Sum (digits (1040,10)) = (Sum <%0,4,0%>) + (Sum <%1%>) by Th432, AFINSQ_2:55
.= ((Sum <%0,4%>) + (Sum <%0%>)) + (Sum <%1%>) by AFINSQ_2:55
.= ((0 + 4) + (Sum <%0%>)) + (Sum <%1%>) by AFINSQ_2:54
.= ((0 + 4) + 0) + (Sum <%1%>) by AFINSQ_2:53
.= 4 + 1 by AFINSQ_2:53
.= 5 ; :: thesis: verum