
theorem Th621:
Sum digits(2920,10) = 13
proof
  thus Sum digits(2920,10) = Sum <%0,2,9%>+Sum <%2%> by Th620,AFINSQ_2:55
  .= Sum <%0,2%> + Sum <%9%> + Sum <%2%> by AFINSQ_2:55
  .= (0 + 2) + Sum <%9%> + Sum <%2%> by AFINSQ_2:54
  .= (0 + 2 + 9) + Sum <%2%> by AFINSQ_2:53
  .= 11 + 2 by AFINSQ_2:53
  .= 13;
end;
