thus Sum (digits (1298,10)) = (Sum <%8,9,2%>) + (Sum <%1%>) by Th828, AFINSQ_2:55
.= ((Sum <%8,9%>) + (Sum <%2%>)) + (Sum <%1%>) by AFINSQ_2:55
.= ((8 + 9) + (Sum <%2%>)) + (Sum <%1%>) by AFINSQ_2:54
.= ((8 + 9) + 2) + (Sum <%1%>) by AFINSQ_2:53
.= 19 + 1 by AFINSQ_2:53
.= 20 ; :: thesis: verum