Constraints

["İbrahim Gökhan]

Sorry about my writing is not very good.

We may decide:
1) Which is nicer?
2) Which is easier?
3) Which is more readable and understandable?
4) Which is more automatic?
etc.

We can do all these stuff by using "boost::enable_if", 
"boost::mpl::and_", "boost::mpl::or_" in c++, but it is really 
hard to write, read and understand.

The question is: "Can we do it better?"

You can choose one of below: