thus Sum (digits (2375,10)) = (Sum <%5,7,3%>) + (Sum <%2%>) by Th1199, AFINSQ_2:55
.= ((Sum <%5,7%>) + (Sum <%3%>)) + (Sum <%2%>) by AFINSQ_2:55
.= ((5 + 7) + (Sum <%3%>)) + (Sum <%2%>) by AFINSQ_2:54
.= ((5 + 7) + 3) + (Sum <%2%>) by AFINSQ_2:53
.= 15 + 2 by AFINSQ_2:53
.= 17 ; :: thesis: verum