
theorem Th209:
Sum digits(368,10) = 17
proof
  thus Sum digits(368,10) = Sum <%8,6%>+Sum <%3%> by Th208,AFINSQ_2:55
  .= (8 + 6) + Sum <%3%> by AFINSQ_2:54
  .= 14 + 3 by AFINSQ_2:53
  .= 17;
end;
