thus Sum (digits (1920,10)) = (Sum <%0,2,9%>) + (Sum <%1%>) by Th520, AFINSQ_2:55
.= ((Sum <%0,2%>) + (Sum <%9%>)) + (Sum <%1%>) by AFINSQ_2:55
.= ((0 + 2) + (Sum <%9%>)) + (Sum <%1%>) by AFINSQ_2:54
.= ((0 + 2) + 9) + (Sum <%1%>) by AFINSQ_2:53
.= 11 + 1 by AFINSQ_2:53
.= 12 ; :: thesis: verum