thus Sum (digits (3780,10)) = (Sum <%0,8,7%>) + (Sum <%3%>) by Th706, AFINSQ_2:55
.= ((Sum <%0,8%>) + (Sum <%7%>)) + (Sum <%3%>) by AFINSQ_2:55
.= ((0 + 8) + (Sum <%7%>)) + (Sum <%3%>) by AFINSQ_2:54
.= ((0 + 8) + 7) + (Sum <%3%>) by AFINSQ_2:53
.= 15 + 3 by AFINSQ_2:53
.= 18 ; :: thesis: verum