thus Sum (digits (2175,10)) = (Sum <%5,7,1%>) + (Sum <%2%>) by Th1187, AFINSQ_2:55
.= ((Sum <%5,7%>) + (Sum <%1%>)) + (Sum <%2%>) by AFINSQ_2:55
.= ((5 + 7) + (Sum <%1%>)) + (Sum <%2%>) by AFINSQ_2:54
.= ((5 + 7) + 1) + (Sum <%2%>) by AFINSQ_2:53
.= 13 + 2 by AFINSQ_2:53
.= 15 ; :: thesis: verum