thus Sum (digits (1958,10)) = (Sum <%8,5,9%>) + (Sum <%1%>) by Th882, AFINSQ_2:55
.= ((Sum <%8,5%>) + (Sum <%9%>)) + (Sum <%1%>) by AFINSQ_2:55
.= ((8 + 5) + (Sum <%9%>)) + (Sum <%1%>) by AFINSQ_2:54
.= ((8 + 5) + 9) + (Sum <%1%>) by AFINSQ_2:53
.= 22 + 1 by AFINSQ_2:53
.= 23 ; :: thesis: verum