thus Sum (digits (1080,10)) = (Sum <%0,8,0%>) + (Sum <%1%>) by Th436, AFINSQ_2:55
.= ((Sum <%0,8%>) + (Sum <%0%>)) + (Sum <%1%>) by AFINSQ_2:55
.= ((0 + 8) + (Sum <%0%>)) + (Sum <%1%>) by AFINSQ_2:54
.= ((0 + 8) + 0) + (Sum <%1%>) by AFINSQ_2:53
.= 8 + 1 by AFINSQ_2:53
.= 9 ; :: thesis: verum