thus Sum (digits (2090,10)) = (Sum <%0,9,0%>) + (Sum <%2%>) by Th892, AFINSQ_2:55
.= ((Sum <%0,9%>) + (Sum <%0%>)) + (Sum <%2%>) by AFINSQ_2:55
.= ((0 + 9) + (Sum <%0%>)) + (Sum <%2%>) by AFINSQ_2:54
.= ((0 + 9) + 0) + (Sum <%2%>) by AFINSQ_2:53
.= 9 + 2 by AFINSQ_2:53
.= 11 ; :: thesis: verum