
theorem Th72:
Sum digits(99,10) = 18
proof
  thus Sum digits(99,10) = Sum <%9%>+Sum <%9%> by Th71,AFINSQ_2:55
  .= 9 + Sum <%9%> by AFINSQ_2:53
  .= 9+9 by AFINSQ_2:53
  .= 18;
end;
