thus Sum (digits (1020,10)) = (Sum <%0,2,0%>) + (Sum <%1%>) by Th430, AFINSQ_2:55
.= ((Sum <%0,2%>) + (Sum <%0%>)) + (Sum <%1%>) by AFINSQ_2:55
.= ((0 + 2) + (Sum <%0%>)) + (Sum <%1%>) by AFINSQ_2:54
.= ((0 + 2) + 0) + (Sum <%1%>) by AFINSQ_2:53
.= 2 + 1 by AFINSQ_2:53
.= 3 ; :: thesis: verum