thus Sum (digits (1518,10)) = (Sum <%8,1,5%>) + (Sum <%1%>) by Th846, AFINSQ_2:55
.= ((Sum <%8,1%>) + (Sum <%5%>)) + (Sum <%1%>) by AFINSQ_2:55
.= ((8 + 1) + (Sum <%5%>)) + (Sum <%1%>) by AFINSQ_2:54
.= ((8 + 1) + 5) + (Sum <%1%>) by AFINSQ_2:53
.= 14 + 1 by AFINSQ_2:53
.= 15 ; :: thesis: verum