keyword in or ∈ for membership checking

[mahrud]

Originally from here:

I think ∈ should be a synonym for in so you can write for x ∈ L do ..., though maybe we can also add new syntactic sugar x in L which calls isMember!

To clarify, there would be two situations for in:

• inside a for clause, acts as "for every element in L"
• outside a for clause, acts as "whether element is a member in L"

[pzinn]

another pair of symbols which are already synonyms, but for which the current syntax doesn't match proper mathematical conventions, is

"∀" => "all",
"∃" => "any",