thus Sum (digits (2750,10)) = (Sum <%0,5,7%>) + (Sum <%2%>) by Th1221, AFINSQ_2:55
.= ((Sum <%0,5%>) + (Sum <%7%>)) + (Sum <%2%>) by AFINSQ_2:55
.= ((0 + 5) + (Sum <%7%>)) + (Sum <%2%>) by AFINSQ_2:54
.= ((0 + 5) + 7) + (Sum <%2%>) by AFINSQ_2:53
.= 12 + 2 by AFINSQ_2:53
.= 14 ; :: thesis: verum