thus Sum (digits (1980,10)) = (Sum <%0,8,9%>) + (Sum <%1%>) by Th526, AFINSQ_2:55
.= ((Sum <%0,8%>) + (Sum <%9%>)) + (Sum <%1%>) by AFINSQ_2:55
.= ((0 + 8) + (Sum <%9%>)) + (Sum <%1%>) by AFINSQ_2:54
.= ((0 + 8) + 9) + (Sum <%1%>) by AFINSQ_2:53
.= 17 + 1 by AFINSQ_2:53
.= 18 ; :: thesis: verum