thus Sum (digits (3220,10)) = (Sum <%0,2,2%>) + (Sum <%3%>) by Th650, AFINSQ_2:55
.= ((Sum <%0,2%>) + (Sum <%2%>)) + (Sum <%3%>) by AFINSQ_2:55
.= ((0 + 2) + (Sum <%2%>)) + (Sum <%3%>) by AFINSQ_2:54
.= ((0 + 2) + 2) + (Sum <%3%>) by AFINSQ_2:53
.= 4 + 3 by AFINSQ_2:53
.= 7 ; :: thesis: verum