thus Sum (digits (1560,10)) = (Sum <%0,6,5%>) + (Sum <%1%>) by Th484, AFINSQ_2:55
.= ((Sum <%0,6%>) + (Sum <%5%>)) + (Sum <%1%>) by AFINSQ_2:55
.= ((0 + 6) + (Sum <%5%>)) + (Sum <%1%>) by AFINSQ_2:54
.= ((0 + 6) + 5) + (Sum <%1%>) by AFINSQ_2:53
.= 11 + 1 by AFINSQ_2:53
.= 12 ; :: thesis: verum