thus Sum (digits (1320,10)) = (Sum <%0,2,3%>) + (Sum <%1%>) by Th460, AFINSQ_2:55
.= ((Sum <%0,2%>) + (Sum <%3%>)) + (Sum <%1%>) by AFINSQ_2:55
.= ((0 + 2) + (Sum <%3%>)) + (Sum <%1%>) by AFINSQ_2:54
.= ((0 + 2) + 3) + (Sum <%1%>) by AFINSQ_2:53
.= 5 + 1 by AFINSQ_2:53
.= 6 ; :: thesis: verum