thus Sum (digits (1254,10)) = (Sum <%4,5,2%>) + (Sum <%1%>) by Th824, AFINSQ_2:55
.= ((Sum <%4,5%>) + (Sum <%2%>)) + (Sum <%1%>) by AFINSQ_2:55
.= ((4 + 5) + (Sum <%2%>)) + (Sum <%1%>) by AFINSQ_2:54
.= ((4 + 5) + 2) + (Sum <%1%>) by AFINSQ_2:53
.= 11 + 1 by AFINSQ_2:53
.= 12 ; :: thesis: verum