thus Sum (digits (2850,10)) = (Sum <%0,5,8%>) + (Sum <%2%>) by Th1227, AFINSQ_2:55
.= ((Sum <%0,5%>) + (Sum <%8%>)) + (Sum <%2%>) by AFINSQ_2:55
.= ((0 + 5) + (Sum <%8%>)) + (Sum <%2%>) by AFINSQ_2:54
.= ((0 + 5) + 8) + (Sum <%2%>) by AFINSQ_2:53
.= 13 + 2 by AFINSQ_2:53
.= 15 ; :: thesis: verum