To Do List

* rewriting "member" function
Currently, member uses the "or" function and also it's not early terminating.
Rewriting member using "foldr" was proposed. And the idea is to promote reusing lemmas for foldr. But then, it won't be early terminating either.
If we rewrite it, I(Duckki) think we might like an early terminating one. What do you think?
At this time, I'm afraid to change the member function, because it is being used so many places. It would be a major change.

