thus Sum (digits (3820,10)) = (Sum <%0,2,8%>) + (Sum <%3%>) by Th710, AFINSQ_2:55
.= ((Sum <%0,2%>) + (Sum <%8%>)) + (Sum <%3%>) by AFINSQ_2:55
.= ((0 + 2) + (Sum <%8%>)) + (Sum <%3%>) by AFINSQ_2:54
.= ((0 + 2) + 8) + (Sum <%3%>) by AFINSQ_2:53
.= 10 + 3 by AFINSQ_2:53
.= 13 ; :: thesis: verum