thus Sum (digits (1475,10)) = (Sum <%5,7,4%>) + (Sum <%1%>) by Th1147, AFINSQ_2:55
.= ((Sum <%5,7%>) + (Sum <%4%>)) + (Sum <%1%>) by AFINSQ_2:55
.= ((5 + 7) + (Sum <%4%>)) + (Sum <%1%>) by AFINSQ_2:54
.= ((5 + 7) + 4) + (Sum <%1%>) by AFINSQ_2:53
.= 16 + 1 by AFINSQ_2:53
.= 17 ; :: thesis: verum