thus Sum (digits (1386,10)) = (Sum <%6,8,3%>) + (Sum <%1%>) by Th834, AFINSQ_2:55
.= ((Sum <%6,8%>) + (Sum <%3%>)) + (Sum <%1%>) by AFINSQ_2:55
.= ((6 + 8) + (Sum <%3%>)) + (Sum <%1%>) by AFINSQ_2:54
.= ((6 + 8) + 3) + (Sum <%1%>) by AFINSQ_2:53
.= 17 + 1 by AFINSQ_2:53
.= 18 ; :: thesis: verum