thus Sum (digits (4225,10)) = (Sum <%5,2,2%>) + (Sum <%4%>) by Th1315, AFINSQ_2:55
.= ((Sum <%5,2%>) + (Sum <%2%>)) + (Sum <%4%>) by AFINSQ_2:55
.= ((5 + 2) + (Sum <%2%>)) + (Sum <%4%>) by AFINSQ_2:54
.= ((5 + 2) + 2) + (Sum <%4%>) by AFINSQ_2:53
.= 9 + 4 by AFINSQ_2:53
.= 13 ; :: thesis: verum