thus Sum (digits (2100,10)) = (Sum <%0,0,1%>) + (Sum <%2%>) by Th538, AFINSQ_2:55
.= ((Sum <%0,0%>) + (Sum <%1%>)) + (Sum <%2%>) by AFINSQ_2:55
.= ((0 + 0) + (Sum <%1%>)) + (Sum <%2%>) by AFINSQ_2:54
.= ((0 + 0) + 1) + (Sum <%2%>) by AFINSQ_2:53
.= 1 + 2 by AFINSQ_2:53
.= 3 ; :: thesis: verum