thus Sum (digits (1250,10)) = (Sum <%0,5,2%>) + (Sum <%1%>) by Th1133, AFINSQ_2:55
.= ((Sum <%0,5%>) + (Sum <%2%>)) + (Sum <%1%>) by AFINSQ_2:55
.= ((0 + 5) + (Sum <%2%>)) + (Sum <%1%>) by AFINSQ_2:54
.= ((0 + 5) + 2) + (Sum <%1%>) by AFINSQ_2:53
.= 7 + 1 by AFINSQ_2:53
.= 8 ; :: thesis: verum