thus Sum (digits (1225,10)) = (Sum <%5,2,2%>) + (Sum <%1%>) by Th1131, AFINSQ_2:55
.= ((Sum <%5,2%>) + (Sum <%2%>)) + (Sum <%1%>) by AFINSQ_2:55
.= ((5 + 2) + (Sum <%2%>)) + (Sum <%1%>) by AFINSQ_2:54
.= ((5 + 2) + 2) + (Sum <%1%>) by AFINSQ_2:53
.= 9 + 1 by AFINSQ_2:53
.= 10 ; :: thesis: verum