thus Sum (digits (1820,10)) = (Sum <%0,2,8%>) + (Sum <%1%>) by Th510, AFINSQ_2:55
.= ((Sum <%0,2%>) + (Sum <%8%>)) + (Sum <%1%>) by AFINSQ_2:55
.= ((0 + 2) + (Sum <%8%>)) + (Sum <%1%>) by AFINSQ_2:54
.= ((0 + 2) + 8) + (Sum <%1%>) by AFINSQ_2:53
.= 10 + 1 by AFINSQ_2:53
.= 11 ; :: thesis: verum