A membership test asks whether a value or property is present in a collection. For strings, a related way to check for a substring is `find()`: it returns the starting index when the substring is present and `-1` when it is absent. For example, `"some_string".find("ing")` returns `8`, while `"who".find("you")` returns `-1`. section 8.1
Searching a string
With `who = "me myself and I"`, `who.find("and")` returns `10`; `who.find("you")` returns `-1`. Use `rfind()` to locate a matching substring nearest the end instead. section 8.1
