thus Sum (digits (1880,10)) = (Sum <%0,8,8%>) + (Sum <%1%>) by Th516, AFINSQ_2:55
.= ((Sum <%0,8%>) + (Sum <%8%>)) + (Sum <%1%>) by AFINSQ_2:55
.= ((0 + 8) + (Sum <%8%>)) + (Sum <%1%>) by AFINSQ_2:54
.= ((0 + 8) + 8) + (Sum <%1%>) by AFINSQ_2:53
.= 16 + 1 by AFINSQ_2:53
.= 17 ; :: thesis: verum