thus Sum (digits (4125,10)) = (Sum <%5,2,1%>) + (Sum <%4%>) by Th1307, AFINSQ_2:55
.= ((Sum <%5,2%>) + (Sum <%1%>)) + (Sum <%4%>) by AFINSQ_2:55
.= ((5 + 2) + (Sum <%1%>)) + (Sum <%4%>) by AFINSQ_2:54
.= ((5 + 2) + 1) + (Sum <%4%>) by AFINSQ_2:53
.= 8 + 4 by AFINSQ_2:53
.= 12 ; :: thesis: verum