
theorem Th389:
Sum digits(580,10) = 13
proof
  thus Sum digits(580,10) = Sum <%0,8%>+Sum <%5%> by Th388,AFINSQ_2:55
  .= (0 + 8) + Sum <%5%> by AFINSQ_2:54
  .= 8 + 5 by AFINSQ_2:53
  .= 13;
end;
