thus Sum (digits (250,10)) = (Sum <%0,5%>) + (Sum <%2%>) by Th1083, AFINSQ_2:55
.= (0 + 5) + (Sum <%2%>) by AFINSQ_2:54
.= 5 + 2 by AFINSQ_2:53
.= 7 ; :: thesis: verum