thus Sum (digits (1420,10)) = (Sum <%0,2,4%>) + (Sum <%1%>) by Th470, AFINSQ_2:55
.= ((Sum <%0,2%>) + (Sum <%4%>)) + (Sum <%1%>) by AFINSQ_2:55
.= ((0 + 2) + (Sum <%4%>)) + (Sum <%1%>) by AFINSQ_2:54
.= ((0 + 2) + 4) + (Sum <%1%>) by AFINSQ_2:53
.= 6 + 1 by AFINSQ_2:53
.= 7 ; :: thesis: verum