thus Sum (digits (1914,10)) = (Sum <%4,1,9%>) + (Sum <%1%>) by Th878, AFINSQ_2:55
.= ((Sum <%4,1%>) + (Sum <%9%>)) + (Sum <%1%>) by AFINSQ_2:55
.= ((4 + 1) + (Sum <%9%>)) + (Sum <%1%>) by AFINSQ_2:54
.= ((4 + 1) + 9) + (Sum <%1%>) by AFINSQ_2:53
.= 14 + 1 by AFINSQ_2:53
.= 15 ; :: thesis: verum