thus Sum (digits (1375,10)) = (Sum <%5,7,3%>) + (Sum <%1%>) by Th1141, AFINSQ_2:55
.= ((Sum <%5,7%>) + (Sum <%3%>)) + (Sum <%1%>) by AFINSQ_2:55
.= ((5 + 7) + (Sum <%3%>)) + (Sum <%1%>) by AFINSQ_2:54
.= ((5 + 7) + 3) + (Sum <%1%>) by AFINSQ_2:53
.= 15 + 1 by AFINSQ_2:53
.= 16 ; :: thesis: verum