thus Sum (digits (1826,10)) = (Sum <%6,2,8%>) + (Sum <%1%>) by Th870, AFINSQ_2:55
.= ((Sum <%6,2%>) + (Sum <%8%>)) + (Sum <%1%>) by AFINSQ_2:55
.= ((6 + 2) + (Sum <%8%>)) + (Sum <%1%>) by AFINSQ_2:54
.= ((6 + 2) + 8) + (Sum <%1%>) by AFINSQ_2:53
.= 16 + 1 by AFINSQ_2:53
.= 17 ; :: thesis: verum