thus Sum (digits (4425,10)) = (Sum <%5,2,4%>) + (Sum <%4%>) by Th1331, AFINSQ_2:55
.= ((Sum <%5,2%>) + (Sum <%4%>)) + (Sum <%4%>) by AFINSQ_2:55
.= ((5 + 2) + (Sum <%4%>)) + (Sum <%4%>) by AFINSQ_2:54
.= ((5 + 2) + 4) + (Sum <%4%>) by AFINSQ_2:53
.= 11 + 4 by AFINSQ_2:53
.= 15 ; :: thesis: verum