thus Sum (digits (1050,10)) = (Sum <%0,5,0%>) + (Sum <%1%>) by Th1123, AFINSQ_2:55
.= ((Sum <%0,5%>) + (Sum <%0%>)) + (Sum <%1%>) by AFINSQ_2:55
.= ((0 + 5) + (Sum <%0%>)) + (Sum <%1%>) by AFINSQ_2:54
.= ((0 + 5) + 0) + (Sum <%1%>) by AFINSQ_2:53
.= 5 + 1 by AFINSQ_2:53
.= 6 ; :: thesis: verum