
theorem Th661:
Sum digits(3320,10) = 8
proof
  thus Sum digits(3320,10) = Sum <%0,2,3%>+Sum <%3%> by Th660,AFINSQ_2:55
  .= Sum <%0,2%> + Sum <%3%> + Sum <%3%> by AFINSQ_2:55
  .= (0 + 2) + Sum <%3%> + Sum <%3%> by AFINSQ_2:54
  .= (0 + 2 + 3) + Sum <%3%> by AFINSQ_2:53
  .= 5 + 3 by AFINSQ_2:53
  .= 8;
end;
