thus Sum (digits (2920,10)) = (Sum <%0,2,9%>) + (Sum <%2%>) by Th620, AFINSQ_2:55
.= ((Sum <%0,2%>) + (Sum <%9%>)) + (Sum <%2%>) by AFINSQ_2:55
.= ((0 + 2) + (Sum <%9%>)) + (Sum <%2%>) by AFINSQ_2:54
.= ((0 + 2) + 9) + (Sum <%2%>) by AFINSQ_2:53
.= 11 + 2 by AFINSQ_2:53
.= 13 ; :: thesis: verum