thus Sum (digits (4375,10)) = (Sum <%5,7,3%>) + (Sum <%4%>) by Th1327, AFINSQ_2:55
.= ((Sum <%5,7%>) + (Sum <%3%>)) + (Sum <%4%>) by AFINSQ_2:55
.= ((5 + 7) + (Sum <%3%>)) + (Sum <%4%>) by AFINSQ_2:54
.= ((5 + 7) + 3) + (Sum <%4%>) by AFINSQ_2:53
.= 15 + 4 by AFINSQ_2:53
.= 19 ; :: thesis: verum