thus Sum (digits (3525,10)) = (Sum <%5,2,5%>) + (Sum <%3%>) by Th1267, AFINSQ_2:55
.= ((Sum <%5,2%>) + (Sum <%5%>)) + (Sum <%3%>) by AFINSQ_2:55
.= ((5 + 2) + (Sum <%5%>)) + (Sum <%3%>) by AFINSQ_2:54
.= ((5 + 2) + 5) + (Sum <%3%>) by AFINSQ_2:53
.= 12 + 3 by AFINSQ_2:53
.= 15 ; :: thesis: verum