thus Sum (digits (3375,10)) = (Sum <%5,7,3%>) + (Sum <%3%>) by Th1259, AFINSQ_2:55
.= ((Sum <%5,7%>) + (Sum <%3%>)) + (Sum <%3%>) by AFINSQ_2:55
.= ((5 + 7) + (Sum <%3%>)) + (Sum <%3%>) by AFINSQ_2:54
.= ((5 + 7) + 3) + (Sum <%3%>) by AFINSQ_2:53
.= 15 + 3 by AFINSQ_2:53
.= 18 ; :: thesis: verum