thus Sum (digits (2550,10)) = (Sum <%0,5,5%>) + (Sum <%2%>) by Th1209, AFINSQ_2:55
.= ((Sum <%0,5%>) + (Sum <%5%>)) + (Sum <%2%>) by AFINSQ_2:55
.= ((0 + 5) + (Sum <%5%>)) + (Sum <%2%>) by AFINSQ_2:54
.= ((0 + 5) + 5) + (Sum <%2%>) by AFINSQ_2:53
.= 10 + 2 by AFINSQ_2:53
.= 12 ; :: thesis: verum