
theorem Th338:
Sum digits(684,10) = 18
proof
  thus Sum digits(684,10) = Sum <%4,8%>+Sum <%6%> by Th337,AFINSQ_2:55
  .= (4 + 8) + Sum <%6%> by AFINSQ_2:54
  .= 12 + 6 by AFINSQ_2:53
  .= 18;
end;
