thus Sum (digits (2376,10)) = (Sum <%6,7,3%>) + (Sum <%2%>) by Th916, AFINSQ_2:55
.= ((Sum <%6,7%>) + (Sum <%3%>)) + (Sum <%2%>) by AFINSQ_2:55
.= ((6 + 7) + (Sum <%3%>)) + (Sum <%2%>) by AFINSQ_2:54
.= ((6 + 7) + 3) + (Sum <%2%>) by AFINSQ_2:53
.= 16 + 2 by AFINSQ_2:53
.= 18 ; :: thesis: verum