thus Sum (digits (1660,10)) = (Sum <%0,6,6%>) + (Sum <%1%>) by Th494, AFINSQ_2:55
.= ((Sum <%0,6%>) + (Sum <%6%>)) + (Sum <%1%>) by AFINSQ_2:55
.= ((0 + 6) + (Sum <%6%>)) + (Sum <%1%>) by AFINSQ_2:54
.= ((0 + 6) + 6) + (Sum <%1%>) by AFINSQ_2:53
.= 12 + 1 by AFINSQ_2:53
.= 13 ; :: thesis: verum