thus Sum (digits (1276,10)) = (Sum <%6,7,2%>) + (Sum <%1%>) by Th826, AFINSQ_2:55
.= ((Sum <%6,7%>) + (Sum <%2%>)) + (Sum <%1%>) by AFINSQ_2:55
.= ((6 + 7) + (Sum <%2%>)) + (Sum <%1%>) by AFINSQ_2:54
.= ((6 + 7) + 2) + (Sum <%1%>) by AFINSQ_2:53
.= 15 + 1 by AFINSQ_2:53
.= 16 ; :: thesis: verum