thus Sum (digits (4175,10)) = (Sum <%5,7,1%>) + (Sum <%4%>) by Th1311, AFINSQ_2:55
.= ((Sum <%5,7%>) + (Sum <%1%>)) + (Sum <%4%>) by AFINSQ_2:55
.= ((5 + 7) + (Sum <%1%>)) + (Sum <%4%>) by AFINSQ_2:54
.= ((5 + 7) + 1) + (Sum <%4%>) by AFINSQ_2:53
.= 13 + 4 by AFINSQ_2:53
.= 17 ; :: thesis: verum