
theorem Th829:
Sum digits(1298,10) = 20
proof
  thus Sum digits(1298,10) = Sum <%8,9,2%>+Sum <%1%> by Th828,AFINSQ_2:55
  .= Sum <%8,9%> + Sum <%2%> + Sum <%1%> by AFINSQ_2:55
  .= (8 + 9) + Sum <%2%> + Sum <%1%> by AFINSQ_2:54
  .= (8 + 9 + 2) + Sum <%1%> by AFINSQ_2:53
  .= 19 + 1 by AFINSQ_2:53
  .= 20;
end;
