thus Sum (digits (775,10)) = (Sum <%5,7%>) + (Sum <%7%>) by Th1107, AFINSQ_2:55
.= (5 + 7) + (Sum <%7%>) by AFINSQ_2:54
.= 12 + 7 by AFINSQ_2:53
.= 19 ; :: thesis: verum