thus Sum (digits (1275,10)) = (Sum <%5,7,2%>) + (Sum <%1%>) by Th1135, AFINSQ_2:55
.= ((Sum <%5,7%>) + (Sum <%2%>)) + (Sum <%1%>) by AFINSQ_2:55
.= ((5 + 7) + (Sum <%2%>)) + (Sum <%1%>) by AFINSQ_2:54
.= ((5 + 7) + 2) + (Sum <%1%>) by AFINSQ_2:53
.= 14 + 1 by AFINSQ_2:53
.= 15 ; :: thesis: verum