thus Sum (digits (1870,10)) = (Sum <%0,7,8%>) + (Sum <%1%>) by Th874, AFINSQ_2:55
.= ((Sum <%0,7%>) + (Sum <%8%>)) + (Sum <%1%>) by AFINSQ_2:55
.= ((0 + 7) + (Sum <%8%>)) + (Sum <%1%>) by AFINSQ_2:54
.= ((0 + 7) + 8) + (Sum <%1%>) by AFINSQ_2:53
.= 15 + 1 by AFINSQ_2:53
.= 16 ; :: thesis: verum