thus Sum (digits (4725,10)) = (Sum <%5,2,7%>) + (Sum <%4%>) by Th1355, AFINSQ_2:55
.= ((Sum <%5,2%>) + (Sum <%7%>)) + (Sum <%4%>) by AFINSQ_2:55
.= ((5 + 2) + (Sum <%7%>)) + (Sum <%4%>) by AFINSQ_2:54
.= ((5 + 2) + 7) + (Sum <%4%>) by AFINSQ_2:53
.= 14 + 4 by AFINSQ_2:53
.= 18 ; :: thesis: verum