
theorem Th427:
Sum digits(980,10) = 17
proof
  thus Sum digits(980,10) = Sum <%0,8%>+Sum <%9%> by Th426,AFINSQ_2:55
  .= (0 + 8) + Sum <%9%> by AFINSQ_2:54
  .= 8 + 9 by AFINSQ_2:53
  .= 17;
end;
