
theorem Th509:
Sum digits(1800,10) = 9
proof
  thus Sum digits(1800,10) = Sum <%0,0,8%>+Sum <%1%> by Th508,AFINSQ_2:55
  .= Sum <%0,0%> + Sum <%8%> + Sum <%1%> by AFINSQ_2:55
  .= (0 + 0) + Sum <%8%> + Sum <%1%> by AFINSQ_2:54
  .= (0 + 0 + 8) + Sum <%1%> by AFINSQ_2:53
  .= 8 + 1 by AFINSQ_2:53
  .= 9;
end;
