
theorem Th308:
Sum digits(399,10) = 21
proof
  thus Sum digits(399,10) = Sum <%9,9%>+Sum <%3%> by Th307,AFINSQ_2:55
  .= (9 + 9) + Sum <%3%> by AFINSQ_2:54
  .= 18 + 3 by AFINSQ_2:53
  .= 21;
end;
