thus Sum (digits (1804,10)) = (Sum <%4,0,8%>) + (Sum <%1%>) by Th868, AFINSQ_2:55
.= ((Sum <%4,0%>) + (Sum <%8%>)) + (Sum <%1%>) by AFINSQ_2:55
.= ((4 + 0) + (Sum <%8%>)) + (Sum <%1%>) by AFINSQ_2:54
.= ((4 + 0) + 8) + (Sum <%1%>) by AFINSQ_2:53
.= 12 + 1 by AFINSQ_2:53
.= 13 ; :: thesis: verum