
theorem Th21:
Sum digits(30,10) = 3
proof
  thus Sum digits(30,10) = Sum <%0%>+Sum <%3%> by Th20,AFINSQ_2:55
  .= 0 + Sum <%3%> by AFINSQ_2:53
  .= 3 by AFINSQ_2:53;
end;
