# Membership testing

TODO: ADD MORE

- this exercise relies on testing membership of a value in a collection of values [allergies](../exercise-concepts/allergies.md)
- the `in` keyword, as in `"s" in "string`, allows the user to check membership in the longer string [phone-number](../exercise-concepts/phone-number.md)
- use of the `in` statement is useful to look for an object into a list [binary-search-tree](../exercise-concepts/binary-search-tree.md)
