thus Sum (digits (2025,10)) = (Sum <%5,2,0%>) + (Sum <%2%>) by Th1177, AFINSQ_2:55
.= ((Sum <%5,2%>) + (Sum <%0%>)) + (Sum <%2%>) by AFINSQ_2:55
.= ((5 + 2) + (Sum <%0%>)) + (Sum <%2%>) by AFINSQ_2:54
.= ((5 + 2) + 0) + (Sum <%2%>) by AFINSQ_2:53
.= 7 + 2 by AFINSQ_2:53
.= 9 ; :: thesis: verum