thus Sum (digits (2150,10)) = (Sum <%0,5,1%>) + (Sum <%2%>) by Th1185, AFINSQ_2:55
.= ((Sum <%0,5%>) + (Sum <%1%>)) + (Sum <%2%>) by AFINSQ_2:55
.= ((0 + 5) + (Sum <%1%>)) + (Sum <%2%>) by AFINSQ_2:54
.= ((0 + 5) + 1) + (Sum <%2%>) by AFINSQ_2:53
.= 6 + 2 by AFINSQ_2:53
.= 8 ; :: thesis: verum