
theorem Th964:
Sum digits(621,10) = 9
proof
  thus Sum digits(621,10) = Sum <%1,2%>+Sum <%6%> by Th963,AFINSQ_2:55
  .= (1 + 2) + Sum <%6%> by AFINSQ_2:54
  .= 3 + 6 by AFINSQ_2:53
  .= 9;
end;
