
theorem Th990:
Sum digits(966,10) = 21
proof
  thus Sum digits(966,10) = Sum <%6,6%>+Sum <%9%> by Th989,AFINSQ_2:55
  .= (6 + 6) + Sum <%9%> by AFINSQ_2:54
  .= 12 + 9 by AFINSQ_2:53
  .= 21;
end;
