thus Sum (digits (3940,10)) = (Sum <%0,4,9%>) + (Sum <%3%>) by Th722, AFINSQ_2:55
.= ((Sum <%0,4%>) + (Sum <%9%>)) + (Sum <%3%>) by AFINSQ_2:55
.= ((0 + 4) + (Sum <%9%>)) + (Sum <%3%>) by AFINSQ_2:54
.= ((0 + 4) + 9) + (Sum <%3%>) by AFINSQ_2:53
.= 13 + 3 by AFINSQ_2:53
.= 16 ; :: thesis: verum