thus Sum (digits (2520,10)) = (Sum <%0,2,5%>) + (Sum <%2%>) by Th580, AFINSQ_2:55
.= ((Sum <%0,2%>) + (Sum <%5%>)) + (Sum <%2%>) by AFINSQ_2:55
.= ((0 + 2) + (Sum <%5%>)) + (Sum <%2%>) by AFINSQ_2:54
.= ((0 + 2) + 5) + (Sum <%2%>) by AFINSQ_2:53
.= 7 + 2 by AFINSQ_2:53
.= 9 ; :: thesis: verum