
theorem Th501:
Sum digits(1720,10) = 10
proof
  thus Sum digits(1720,10) = Sum <%0,2,7%>+Sum <%1%> by Th500,AFINSQ_2:55
  .= Sum <%0,2%> + Sum <%7%> + Sum <%1%> by AFINSQ_2:55
  .= (0 + 2) + Sum <%7%> + Sum <%1%> by AFINSQ_2:54
  .= (0 + 2 + 7) + Sum <%1%> by AFINSQ_2:53
  .= 9 + 1 by AFINSQ_2:53
  .= 10;
end;
