thus Sum (digits (1380,10)) = (Sum <%0,8,3%>) + (Sum <%1%>) by Th466, AFINSQ_2:55
.= ((Sum <%0,8%>) + (Sum <%3%>)) + (Sum <%1%>) by AFINSQ_2:55
.= ((0 + 8) + (Sum <%3%>)) + (Sum <%1%>) by AFINSQ_2:54
.= ((0 + 8) + 3) + (Sum <%1%>) by AFINSQ_2:53
.= 11 + 1 by AFINSQ_2:53
.= 12 ; :: thesis: verum