thus Sum (digits (3540,10)) = (Sum <%0,4,5%>) + (Sum <%3%>) by Th682, AFINSQ_2:55
.= ((Sum <%0,4%>) + (Sum <%5%>)) + (Sum <%3%>) by AFINSQ_2:55
.= ((0 + 4) + (Sum <%5%>)) + (Sum <%3%>) by AFINSQ_2:54
.= ((0 + 4) + 5) + (Sum <%3%>) by AFINSQ_2:53
.= 9 + 3 by AFINSQ_2:53
.= 12 ; :: thesis: verum