
theorem Th122:
Sum digits(169,10) = 16
proof
  thus Sum digits(169,10) = Sum <%9,6%>+Sum <%1%> by Th121,AFINSQ_2:55
  .= (9 + 6) + Sum <%1%> by AFINSQ_2:54
  .= 15 + 1 by AFINSQ_2:53
  .= 16;
end;
