thus Sum (digits (2020,10)) = (Sum <%0,2,0%>) + (Sum <%2%>) by Th530, AFINSQ_2:55
.= ((Sum <%0,2%>) + (Sum <%0%>)) + (Sum <%2%>) by AFINSQ_2:55
.= ((0 + 2) + (Sum <%0%>)) + (Sum <%2%>) by AFINSQ_2:54
.= ((0 + 2) + 0) + (Sum <%2%>) by AFINSQ_2:53
.= 2 + 2 by AFINSQ_2:53
.= 4 ; :: thesis: verum