Add Check List Membership as an elixir til

elixir/check-list-membership.md

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+# Check List Membership
+
+You can use the [`in` operator](https://hexdocs.pm/elixir/operators.html) to
+check if something appears in a list. This is a handy way of checking if a
+variable is one of a few acceptable or expected values.
+
+For instance, a common DateTime comparison pattern relies on this to check
+if a DateTime is `>=` or `<=` to another DateTime.
+
+```elixir
+{:ok, datetime} = DateTime.from_naive(~N[2016-05-24 13:26:08.003], "Etc/UTC")
+
+DateTime.compare(datetime, DateTime.utc_now()) in [:lt, :eq]
+```
+
+Alternatively, you can check that something does not have membership in a
+list by also including the `not` operator.
+
+```elixir
+DateTime.compare(datetime, DateTime.utc_now()) not in [:lt, :eq]
+```