thus Sum (digits (1750,10)) = (Sum <%0,5,7%>) + (Sum <%1%>) by Th1161, AFINSQ_2:55
.= ((Sum <%0,5%>) + (Sum <%7%>)) + (Sum <%1%>) by AFINSQ_2:55
.= ((0 + 5) + (Sum <%7%>)) + (Sum <%1%>) by AFINSQ_2:54
.= ((0 + 5) + 7) + (Sum <%1%>) by AFINSQ_2:53
.= 12 + 1 by AFINSQ_2:53
.= 13 ; :: thesis: verum