thus Sum (digits (3860,10)) = (Sum <%0,6,8%>) + (Sum <%3%>) by Th714, AFINSQ_2:55
.= ((Sum <%0,6%>) + (Sum <%8%>)) + (Sum <%3%>) by AFINSQ_2:55
.= ((0 + 6) + (Sum <%8%>)) + (Sum <%3%>) by AFINSQ_2:54
.= ((0 + 6) + 8) + (Sum <%3%>) by AFINSQ_2:53
.= 14 + 3 by AFINSQ_2:53
.= 17 ; :: thesis: verum