thus Sum (digits (2580,10)) = (Sum <%0,8,5%>) + (Sum <%2%>) by Th586, AFINSQ_2:55
.= ((Sum <%0,8%>) + (Sum <%5%>)) + (Sum <%2%>) by AFINSQ_2:55
.= ((0 + 8) + (Sum <%5%>)) + (Sum <%2%>) by AFINSQ_2:54
.= ((0 + 8) + 5) + (Sum <%2%>) by AFINSQ_2:53
.= 13 + 2 by AFINSQ_2:53
.= 15 ; :: thesis: verum