thus Sum (digits (1075,10)) = (Sum <%5,7,0%>) + (Sum <%1%>) by Th1125, AFINSQ_2:55
.= ((Sum <%5,7%>) + (Sum <%0%>)) + (Sum <%1%>) by AFINSQ_2:55
.= ((5 + 7) + (Sum <%0%>)) + (Sum <%1%>) by AFINSQ_2:54
.= ((5 + 7) + 0) + (Sum <%1%>) by AFINSQ_2:53
.= 12 + 1 by AFINSQ_2:53
.= 13 ; :: thesis: verum