thus Sum (digits (4200,10)) = (Sum <%0,0,2%>) + (Sum <%4%>) by Th1313, AFINSQ_2:55
.= ((Sum <%0,0%>) + (Sum <%2%>)) + (Sum <%4%>) by AFINSQ_2:55
.= ((0 + 0) + (Sum <%2%>)) + (Sum <%4%>) by AFINSQ_2:54
.= ((0 + 0) + 2) + (Sum <%4%>) by AFINSQ_2:53
.= 2 + 4 by AFINSQ_2:53
.= 6 ; :: thesis: verum