thus Sum (digits (2178,10)) = (Sum <%8,7,1%>) + (Sum <%2%>) by Th900, AFINSQ_2:55
.= ((Sum <%8,7%>) + (Sum <%1%>)) + (Sum <%2%>) by AFINSQ_2:55
.= ((8 + 7) + (Sum <%1%>)) + (Sum <%2%>) by AFINSQ_2:54
.= ((8 + 7) + 1) + (Sum <%2%>) by AFINSQ_2:53
.= 16 + 2 by AFINSQ_2:53
.= 18 ; :: thesis: verum